I get my bitcoin address from freebitco.in. So how can I login into my wallet to know about my bitcoins? How to convert bitcoins into dollars and send it in to my prepaid debit mastercard?
Your question shows that you're quite new to Bitcoin in general. I would advise you to do a little light reading about it to understand what bitcoin exactly is. Regardless..
I get my bitcoin address from freebitco.in . So how can I login into my wallet to know about my bitcoins?
If you acquired your wallet from Freebitco.in, you were redirected to Blockchain.info, you can log in there to check your balance. You will only have balance there if you withdrew coins from Freebitco.in in the first place. Otherwise, log into Freebitco.in and withdraw your coins.
Blockchain.info is a web wallet, you'll find all the information you need about it on their website.
How to convert bitcoins into dollars and send it in to my prepaid debit mastercard?
You will first need to sell your coins for fiat currency. There are literally hundreds of online exchanges, or people you could find locally to sell to on Localbitcoins. Here's a nice guide to get you started.
After you've read the guide, signed up for an exchange, and sold your coins, you can have them transfered to your bank account and use the money with your prepaid card.
Congratulations on starting with Bitcoin, I would recommend you not to cash out, and instead of converting your coins to fiat, try finding places where you can spend your bitcoin instead.