You will have received a password in email. Never give it to anyone. Never. Not to anyone. Even me!. I do check my logs to see who uses it.
Find my server in the game browser, and get the IP number; it will be 4 numbers with dots, and will look like aa.bb.cc.dd (example: 188.8.131.52), and will be different every few days. Then hit tab (or tilde), and type: "open aa.bb.cc.dd?password=youradminpassword" (no quotes, substitute the password you were sent for youradminpassword). This is exactly the same way to join passworded games with the game password.
Admins on my server by invitation only. Everybody wants to be admin, usually so they can have some special privilege. Well, start a server, and you will have full and total control over it. I don't give out admin just for asking, and especially repeated asking is quite annoying. The answer is already, NO. I already have several people that admin on my server. They got that way by invitation, not by begging. (This doesn't mean don't *ever* ask, other wise how would I know? Just please only ask once; EMail is best.)
The admins are there to help the game go smoothly, and to keep down trouble with problem players. They are there to help you have a good time in the server. The admins are not your servants; they are not like wait staff in a restaurant to do your bidding. If you're a bother, we'll start ignoring you. If you get troublesome, well, see the server rules.
Temp Admin will be given for many of the responsible players that I have know and trust. This will be admin privilege for 2-3 weeks at a time, and will rotate with maybe 2-3 temp admins at any one time.
Needless to say, once you have an admin password from me, don't ever tell it to anyone else. Including me, even if I ask for it.