Can you give me some advice about football trial?
Practice at least 1-2 hours / day if you have time, and run as much as you can in advance to build up your endurance. We call it "soccer" on my side of the pond, however, you can never be too fast at running or have too much endurance.
Also stretch regularly, because I noticed when I played a lot that if I was more flexible, I ran faster.
If you're talking tryouts - drink LOTS of water - make sure you're physically fit and be ready to run lap after lap in the hot sun...take care !!