Also remember to use strong passwords. I used to use one password for every single account I've made on the internet, including 000webhost - which got hacked and their password database is available somewhere for download (they kept their password in plain text). Now I've learned a lesson and I'm using a different, strong 24 characters long password for all the websites - if you value your accounts, especially your Steam one, consider installing KeePass and create strong password for each online account you own. You'll only need to create and remember a strong password for this program (it keeps your passwords encrypted and as safe as possible).
Of course, first of all get rid of all the viruses/keyloggers/malware from your computer. If you're unsure how to do it, ask a friend who's more "into computers", I'm sure he'll help you with that. Just get him a good bottle of beer