If you want to buy with cash, you can find someone in your area willing to accept cash on
https://localbitcoins.com/. If you're in the US, you can also use BitInstant:
https://bitinstant.com.
If you're in the US and you have a checking or savings account, you can sign up at Coinbase (
https://coinbase.com/?r=51a5b0d3867ba8db8e00000c), link your account, and buy Bitcoins directly, then send them to your Blockchain.info wallet.
You can also get small amounts of BTC from various faucets and free sites, but the amounts are too small to buy anything with. They can help cover transaction fees, though, and if nothing else it's a free way to get some Bitcoins into your wallet just so you don't have a zero balance. There are lots and lots of free BTC sites. Here's a site that includes timers and links for a variety of free sites:
http://coinfaucets.tk/.
I have a more elaborate write-up of some options for acquiring Bitcoins, including detailed walk-throughs for setting up on Coinbase or BitInstant, here:
http://www.bit-buddy.com/acquiring-bitcoins.html.