Good luck for tonight!
Let them know its your first WL Line. Main things i'd say:
1) On the line its easy to get caught ball watching, keep focused on staying in line with the second last defender, even when the ball is down the far end from you.
2) You always have much more time than you think you will. Take a moment to make sure you're sure of your decision then put your flag up
3) Try and make as much eye contact with the referee as possible. That helps to avoid you crossing - where you go one way and the ref goes the other.
4) one of the tips I got given which works for me is to verbalise what you are seeing - I'm often talking to myself saying "5 on" (defender number 5 is playing the attacker onside) or "10 off" (attacking 10 is offside). I also use "Goalkick, not live" to indicate to myself that the attacker can't be offside from the GK, then "live" when the ball makes contact with another player.
5) Generally in this league, the No 3 is the left back and will be taking the throw ins in your area. Make a note of their names and use them "just there please <name>, thank you" it goes a long way to keep them on side.
The bit that gets most new AR's is all of the stuff that surrounds the game - not the actual giving of decisions when the game is under way.
1) Turn up in Jacket, Shirt and Tie
2) Ref will do a pre-match, often on the pitch, sometimes in the changing room, listen to him and if there are any things you don't understand, ask him to clarify - better done there, then out of the FOP with loads of people shouting at you.
3) Team sheets will be given to the officials 45 mins before K/O - you shouldn't have to worry as most likely you'll be the "non-senior" AR - whilst the Ref and Senior AR sort out the sheets, you can make sure the match balls are fine and at the right pressure
4) You'll take one team and the other AR will take the other, you do the equipment check, same as any game, no jewellery, sock tape and under armour all the right colour
5) Once you are happy, you head over to the referee and he'll lead the teams out, where they'll do the respect handshake in front of one of the stands
6) After respect handshake, you and other AR will "break" to check the nets, give them a check, then wait until both you and the other AR are ready, normally indicated by a thumbs up out beside you. That way you head back and arrive at the middle at roughly the same time
7) Witness the toss, then head over to your line - it looks a little more professional to backpedal to your line than turning and running forwards
8) When ref blows for halftime, run over to him and stand by his side
9) at end of halftime, you recheck the nets, before heading directly over to your line
10) when ref blows for full-time do same as point 8
This is the first league you'll do where the spectators pay to get in, some of them will position themselves behind you to try to bait you, or engage you in discussion, try and block them out and focus on your game.
The ref and the other AR are both top guys, they'll look after you!