I think you are slightly misunderstanding, but you are on a logical path. You need a mining application/program (I suggest BFGminer, if you are on windows then consider guiminer). So what you have is your own bitcoin instance which you could connect to and mine from... And if you some how manage to get a high diff share from that computer possible in a few thousand years you would get a block. (Hence mining in a pool)

A quick tl;dr because it looks like you are using a nix box:

• Make sure you have git as well as your general building tools.

• Then

  git clone https://github.com/luke-jr/bfgminer.git
  

• To compile:

  ./autogen
  ./configure --enable-cpumining
  make
  

The “readme” is here: https://github.com/luke-jr/bfgminer/blob/bfgminer/README

Once you have finished making BFGminer, go over to slush pool and log in. Then, in that right section,is a list of workers under the heading "Workers". It’ll bring you to a page asking you for a login suffix (it is the way to designate each unique computer mining but having it tied to your account) you also are asked to use a password for that miner. I suggest not using your log in password since it is displayed without any encryption on your main log in thread.

Once you entered your login suffix (For example If my username was "Joe" and my computer was called "Feet" (sorry inside joke with some friends have to use it), my suffix would be ".feet". My entire worker name would be "Joe.feet". My selected password would be "toes" for this example.

After you made a suffix and a password, you click "Save" and you are now back at the main portal. Just again for the sake of repeating. My new worker (also called miner or instance depending on the pool) would be named "Joe.feet" and the password would be "toes"

So "Joe.feet" would be the username I would enter in my mining appplication (BFGMiner). the password would be password I used (in my example "toes").

Now that you have created both a miner and have built BFGminer, figure out if you are on SSE2 or SSE4... once you know which SSE you are using, you can then start mining on slush's pool by using this command (make sure you recurse in to the directory that bfgminer is at). (fyi you could make it a bash script too) (run it in screen as well)

If your on SSE2 run this command other wise i will have the other command a few lines below (again use screen... the command is simply typed screen. You then hit enter twice.)

Make sure you are recursed in to the folder that BFGminer is in and run this code. I am assuming my username on slush pool is "Joe" my worker is called "feet" and my worker password is "toes".

SSE2 version of command

./bfgminer -a sse2_64 -o stratum+tcp://stratum.bitcoin.cz:3333 -u Joe.feet -p toes

SSE4 version of the command

./bfgminer -a sse4_64 -o stratum+tcp://stratum.bitcoin.cz:3333 -u Joe.feet -p toes

After you have done those steps you should see your miner come up to a screen where it says it was connected. At that point you are mining. Please be aware that you have equipment that will probably cost more to run in a day than you will make in profits.

Assessment

I think you are slightly misunderstanding some things, but basically you are already on the right path.

1. Wallet

   I signed up for a bitcoin wallet (coinbase) where I am given an id.

Great.

When you want to manage your wallet (send and receive Bitcoin, create new wallets etc.) on your local machine, that’s a must. On a side-note: in case you need something easier (meaning: with a GUI), you should take a look at the multitude of desktop clients which bitcoin.org points to; for example: Bitcoin-QT.

2. Pool

   I joined a pool (Slush's Pool) where I verified my email and added my private? id from my wallet. They registered a first worker for me:

   login: myusername.worker1  
   password: something given
   

That’s perfect.

3. Miner

   (my hardware doing maths), Xeon processor …

Well, you didn’t mention any mining software yet – and that’s where you took the wrong turn.

Filling the gap with Mining Software

Now that you know that your “Miner” needs “Mining software” to be able to actually mine things like Bitcoin, let’s take a look at what you need to start your own mining efforts…

Since you’ve mentioned you’re using the Ubuntu operating system, you could simply dive into the Ubuntu Software Center to download and download one of the ready-build miners:

• cgminer

  In case you prefer to create your own build from the sourcecode, check the cgminer GitHub repository.

  The easier way is to install a ready-made build from the Software Center, or by entering the following commands in a terminal window:

   apt-get update
   sudo apt-get install cgminer
  

  Or you simply install “cgminer” via the Ubuntu Software Center:

  

• bfgminer

  In case you prefer to create your own build from the sourcecode, check the bfgminer GitHub repository.

  The easier way is to install a ready-made build by entering the following commands in a terminal window:

   apt-get update
   sudo apt-get install bfgminer
  

  Or you simply install “bfgminer” via the Ubuntu Software Center:

  

  You can also create your own build from the sourcecode in case you prefer that. To do so, check the bfgminer GitHub repository.

• CPU Miners

  Personally, I would not recommend using CPU-only miners because they tend to be slower compared to the above mining software. The reason is simple: CPU miners ignore the potential advantage your GPU (graphic card) might offer. Having said that…

  In case you prefer to create your own build from the sourcecode, check the cpuminer GitHub repository.

  The easier way is to install a ready-made build by entering the following commands in a terminal window:

   apt-get update
   sudo apt-get install cpuminer
  

  Or you simply install “cpuminer” via the Ubuntu Software Center:

  

  An aside: This mining software project provided the sourcecode-base on which other projects like bfgminer and cgminer were build.

Running the Mining Software

All of the mining software that has been mentioned above comes with dedicated documentation that explains how to compile the software from sourcecode, how to use the compiled software, etc.

But to get you going, I will provide simple examples for both cgminer and bfgminer. I guess you already know how to open a terminal window to start software from a commandline. For the commandline-parameter data, I’m assuming Slush's pool as you indicated in your question, as well as using your example login myusername.worker1 and the example password mypassword.

• cgminer

  cgminer -o stratum+tcp://stratum.theminingsite.com:3333 -u theusername -p thepassword
  

• bfgminer

  bfgminer -o stratum+tcp://stratum.theminingsite.com:3333 -u theusername -p thepassword
  

• cpuminer

  cpuminer -o stratum+tcp://stratum.theminingsite.com:3333 -u theusername -p thepassword
  

Since most command-line parameters will strongly depend on your hardware, you’ll have to read the docs to see if you can tune things towards higher speeds. In case of doubt, --help offers an initial heads-up on potential tuning options of the individual mining software.

That's it… as soon as you’ve got your mining software installed, up and running, you’ll be mining Bitcoin (or alike crypto-currency).

Good Luck – you'll need it

Unless “the Bitcoin fairy” treats you gently, mining Bitcoin using average computer hardware will produce more costs than profits. In the unlikely case you make “profit”, don’t expect hundreds of Bitcoins… rather think in a realistic range of a few Satochi.

Generally, mining is an electricity-vampire. In other words: you can trust in the fact that mining will increase your energy bill. The chance to make profit will depend on a whole lot of luck, and the actual hashing speed you achieve with your mining rig.

If you really want to get your hands dirty and make profit from mining, you’ll have to consider investing in dedicated hardware (ASICs and/or FPGAs) to achieve higher hashing speeds – which can be rather inexpensive, depending on what hashing speeds you want/need/expect. One thing is for sure: the more hashing speed you trying to achieve, the more you'll find yourself buying more and/or better hardware – which will again will increase your energy bill proportionally. and only you can decide where you draw the fine line between “hobby” and “investment”.

In the end, the amount of cash you’ll have to pay for electricity will have the strongest negative impact on your mining efforts and your energy bill will ultimately dictate your break-even point.