First things first, get yourself to the CTF arena. Taxi's generally have a blorp, but if you ask on the talker someone is sure to give you a lift.
Once there find your team mates in the room with the big rock. 'help here' will give you a run-down of the available commands for the room. You can 'read rock' to see the current teams that have been made, or you can create a team yourself. You'll need to get the team leader to invite you in to their team.
While CTF has teams, and team rooms, these are to distinguish between differing team scores and general team tactics. Wizards might consist of three Teams, one for focusing on casting, one for focusing on flag running, and another for general melee combat. There is no limit to the number of teams you can have. What's important to note is that "Teams" are distinct from "Groups". In CTF you can still form groups just like outside of CTF, and grouping in this way maintains the usual functions for group hunting. So not only do you need to join your CTF Team, but you'll probably need to get into the right group.
Teams designate which Team room you will join, which will give you access to the racks provided (which are chosen by the team leader) for that Team room. Meanwhile your Group will generaly be who you stick besides to fulfil whatever mission you're on. Groups are most important where things like area effect damage from spells are concerned. So, this might mean a casting Wizard group could have a flag runner in the group who doesn't travel around with the group. That way they can move safely through a room with their casting group mates without worrying about getting burned to death by magical shellfire.
You'll need to setup alias's designed to equip you for the start of the game. This will need to be pre match setup in the days ahead of CTF. Get into the CTF arena with a friend and sort out your alias's to get what you need from the racks you need. You might want to ask around for others to provide you with an alias to get you started, and tweak it from there.
I suggest for your first game to just group up with a more experienced player and let them drag you around. Meanwhile you can focus on more simple tasks such as casting, or melee support. This little guide is not meant to go into broader tactics or gameplay.
Play to win.You'll enjoy yourself more, you'll cause others to enjoy themselves more. More fun for all.