1. To sell some Ripples I need to give trust to bitstamp at first ?
If you're going to sell them for bitstamp/BTC, I'd suggest you open an account at Bitstamp and extend trust to them. Otherwise, you can't withdraw them.
2. How to withdraw my 0.2 BTC to my wallet ?
Once you have bitstamp/BTC in Ripple, you would select Ripple deposit from Bitstamp's web site. (Because you are depositing bitcoins into your Bitstamp account.) Once you have the balance off the Ripple network and in your Bitstamp account, you can withdraw it from your Bitstamp account as actual Bitcoins.
So the steps are:
1) Have bitstamp/BTC on the Ripple network.
2) Use Bistamp's web site to deposit these into your Bitstamp account.
3) Withdraw these BTC from your Bitstamp account into a Bitcoin account.