Bridgeing and query help

Hi guys,
I am trying to make a simple bridge between wordpress and the forum software MYBB. I have nearly finished but all i have left to do is to get it to set a MYBB cookie when you login. I have two questions:

Q1:
Where does wordpress set the cookie(/start the session)? Is it in the wp-password.php? as this is what i have been editing and it hasn’t been doing anything so i suspect that is one of my problems.

Q2:
How do i perform a custom query in wordpress?
I have been trying this but it hasn’t worked so far:
$mybbuid->query_vars("SELECT 'uid' FROM 'mybb_users' WHERE uid='$lookuid'");
And quite a few variations of that, but it may be because of my problem with the file.

Thanks 🙂 Hope someone can help me with this.