If you haven't done so already, register with parkrun (
http://www.parkrun.org.uk/register/form/) to receive your unique barcode which can be used at any parkrun event worldwide going forward.
If you're new to a particular event I'd aim to get there for 8:50 at the very latest so you're in time to hear the new runner briefing (informs newcomers what to do, what the course is like, etc.). Then do the run, receive a finish token when you cross over the finish line (take note of the number on the token as it can help the event team should anything go amiss during results processing), head over to the barcode scanning team to get your unique barcode scanned along with the finish token (they both get linked together to give you the right result (the event team then keeps the finish token as they'll need it again for next week's event

)), and head home in a happy mood hoping you've got a great time.
Then repeat it all again the following Saturday in order to beat this time! 48 runs later and you'll be getting your first free technical parkrun running shirt!
By the way you can never be last at a parkrun event as we have to have a tail-runner present, though you shouldn't be anyway if you're aiming for a sub 30 mins time.
Have fun!