Is your first game Open Age or Youth? If Youth, make sure you know what the duration of each half is. U11/U12 - 30, U13/U14 - 35, U15/U16 - 40, U18/OA - 45. I always start my watches before blowing for kick-off, just to make sure they're running. Finally, don't turn up 5 minutes before kick-off - get there with plenty of time to introduce yourself, get the admin done, check the pitch / goals, player kit check etc. Don't worry about any mistakes you might make, as regardless of whether a decision is a mistake or not, one team will invariably be on your back over it - just let that wash over you, and remember - most players / coaches / managers / parents / spectators will not know the LOTG like you do. Most importantly - enjoy yourself!