How do i be a sniper in the royal marine commados (uk)?

i am 15 years old nearly 16 and i want to join the marines, infact i am joining the marines and i want to be a sniper, the physical demands are ok for me i can do 80 press ups in 2 mins and 60 sit ups i 2 mins. i recently ran 3 miles in cross country for the qualifiers at my school my time was 20 mins dead. i just need to know how to be a sniper. do i join infantry are there any special regiments i can join .