Don't be afraid to tell your referee that you're relatively new on the line - there are slightly different duties for each line and they will appreciate knowing that you should be given the lighter workload, and also that you'll need the "full" pre-match briefing. There's no harm in them knowing, although I wouldn't advertise the fact more widely than that if you can help it!
Agree with the majority that a smart tracksuit is the norm for arrival currently. Pack your bag as if you were refereeing the match - the referee may expect you to take notes on cards/goals as if you were in the middle, and on the off chance the other AR is also new to the line/doesn't turn up and the ref gets injured, you may even be called on to step into the middle, so will need your usual whistles and cards for that.
In terms of what to expect, the only thing I'd add is that you need to be aware that your role is to assist the ref. Unlike being in the middle, you'll need to get used to taking the ref's lead - if he makes a confident and quick signal for a throw/corner/gk that you think is wrong, it's often better to simply reinforce that signal rather than contradict him and cause an issue. Some referees will use "penguin flipper" or "downstairs" signals to softly indicate which direction to flag, or will expect you to do the same before signalling, so keep an eye out for that.
And that principal goes further as well. A ref might have a really weird way of doing things, or choose to run right backs, or split who notes what based on the angle of the sun. Even if you think it's weird or counter-productive, it's their game to run how they like.
