There are no requirements about your nationality, skills or need to have a credit card to use Bitcoin. The only major requirement is that you have a smart phone or computer and access to the internet. In terms of earning bitcoin, you may earn it any way you might earn Taka or United states dollars, however you might find few employers will pay you in bitcoin.

There are some services to earn small amounts of bitcoin for simple tasks. A quick search turned up coinworker, but it looks like there are other similar sites if you find that doesn't work.

Hope that answers your question Banda!

1. To improve your knowledge about BTC go to bitcoin.org and en.bitcoin.it (on the latter, Bitcoin Wiki, you can find also Forums & Chats to expand your knowledge).
2. To know about the current price on Bitcoin exchanges go to bitcoinwisdom.com.
3. Some corporations sell computer hardware only in a few countries, example:
   Dell.com (in USA, UK & Canada), NewEgg.com (in USA & Canada). Day by day more corporations are willing to accept BTC as payment, ask them if they do it.
4. To get BTC in your country better try an international exchange site like LocalBitcoins.com, maybe you can find somebody willing to sell BTC and the site provides the info necessary (here's a direct link to their Bangladeshi market).

Hope that this info will give you a quick start into Bitcoin.