Firstly, you need to have a bitcoin wallet. If you don't have one, you can register an account for free at blockchain.info. Then you'll get a bitcoin address (check your blockchain wallet dashboard, receive money, Addresses, Your receiving bitcoin addresses, the "Active" tab), which looks like some long random string. For example, my is "1D8kTXYk2srgGuTrCBP3wu64288GwqKCoS".
Now you are ready to go to some faucet sites and earn free bitcoins. Faucet sites are supported by online advertisements and get their traffic by tipping bitcoins (or other cryptocurrencies) to visitors.There are many faucet sites, usually pay you 100~1000 satoshi (smallest fraction of a Bitcoin that can currently be sent = 0.00000001 BTC) each time you visit. As a beginner, I suggest to try epay.info, which is user friendly and proved to pay. No register needed for epay.info (you can secure your epay account by an email and password later), just go to the URL like
http://epay.info/free-bitcoin/1D8kTXYk2srgGuTrCBP3wu64288GwqKCoS/,
with the bitcoin address "1D8kTXYk2srgGuTrCBP3wu64288GwqKCoS" in it replaced by yours. It shows you a list of faucet sites - just click the button "start browing faucets", you are visiting the faucet sites using epay's rotator. And inside the faucet site page frame, all you need is to find the buttons like "Spin"/"Next"/"Claim" hidden in the ads and solve some captchas (to prove you're a human). After that, you'll get your free bitcoins paid into your epay account.
Once you get paid, you can click "visit next site" (on the right upper corner) to go on your earning. You can withdraw the bitcoins in your epay account to blockchain.info once you reach 0.00013000 BTC (this is not hard since if you play all faucet provided by epay at the time, you can earn 1,543,586 satoshi daily).
Links:
http://blockchain.info (get a wallet)
http://epay.info/ (visit faucet sites and receive tipping)
