You don’t have to warm up on the track – often it is better to do that on the grass; this prevents the track becoming clogged up with joggers and it is also easier on your legs. On your first visit I suggest that you do no more than 8 laps. Run four times round in one of the outside lanes, jogging round the bends and accelerating down the straights, gradually working up to a sprinting speed.
Take a breather, stretch a bit, and then try four laps of running 200m fast and jogging slowly for the reminder. Time yourself over 200m, making sure that you start from the right stagger mark for the lane you are in (ask someone about this if you are in doubt).
Next time you come down – at least three days later, repeat the process but drop the four laps of jogging bends, striding straights and do 8 x 200m.
After three sessions of this you will be ready to tackle running full laps. A good start would be to do 4 x 200m, then 4 x 400m, jogging 200m slowly for your recovery.