As a supplement to the buying process provided by @trollster -
If you would like to earn bitcoin (rather than paying for them directly) there are a few ways to do so. Mining is one example, but it requires purchasing specialized equipment and will likely entail a good deal of research as well. You might choose to sell goods or services in exchange for bitcoin.
A more simple and practical way to earn small amounts of bitcoin is by completing tasks or surveys online. There is an array of websites that offer this form of compensation - which will vary in terms of compensation level, accessibility, reputation, and reliability. From my own experience, I can attest to the fact that www.BitcoinGet.com did pay out (which was my biggest concern). Earning a full bitcoin through this medium alone would take quite a bit of time - but is the only option I can recommend that requires zero capital.
If you do want to earn some bitcoin in that manner, you should still create a bitcoin wallet and an address by following step 1 from @trollster
Welcome to Bitcoin!