Firstly, you should have Bitcoin to do this!(sorry)

Now, you would need a deposit address at poloniex. Copy that, and send bitcoin (from wherever you have it - in some other exchange, I suppose) to this address.

This takes time, anywhere between 2-20 mins(may vary depending upon congestion in blockchain). Once you receive bitcoin at poloniex, you should be able to buy bitshares using those bitcoins.