I know this code:

<form>
NickName: <input type="text" name="nickname"><br>
Password: <input type="password" name="pass">
</form>


How can I make it so it checks something to see if the user and pass is right and if it is the site changes according to the user's info?

You would need to use some type of server side scripting. If you are on a Windows platform, check out:
http://www.outfront.net/spooky/login.htm

There are many ways this can be done. The easyist would be a perl script or you could venture down the .htaccess route. Personaly, I prefer the former.
"Corey Bryant"

Hey, I know you from my site!

Hm, I was not notife that you had replied as well Matrix. Are you following me??

look at other peoples sites with logins. They usually have codes u need. also you need a button tho...lol.