Newbie needs help with website security/user privacy

Hi, I'm new to these forums.

I registered because I'm in desperate need for help.

Can anyone provide me with suggestions on what security methods to use for a site run mostly by php?

I want to help some friends create a website but the website's security is lacking. I can bypass logins simply by manually typing in an address. So for example, I could check out another user's info simply by switching something like
.../userinfo.php?sq=John05 with
.../userinfo.php?sq=SomeoneElse

I have only vague ideas on how to go about fixing these security breaches. Everything I know I only know from reading source codes... but I can only learn so much from that.

Could someone tell me the basics of how to set up secure online transactions?
Or just tell me what vBulletin uses.
Anything would be appreciated.

Thanks in advance!