Creating / adding users

I have had my mac a month now, when I turn it on it boots up straight away. I am the only person using the mac, but how do I password it on startup (presuming it was stolen, so no one could gain access)?

If I do this will it change any settings / or the contents in my home folder?

go to system pref > security > place check mark beside disable automatic login and if you want you can place check mark beside require password to wake this computer from sleep or screen saver

cool! thank you.

There is also the ability to set a firmware password. See this Apple technote.