How does a 13-14 year old join a county running club in North, North-West or East London?

Today, I ran the 100m sprint in 12.47 and I just recovered from a chest infection. My friend in the race with me ran 12.09, we're both 13 going on 14. I'm not asking if these are good times, I'm simply focused on training and improving. We're both considering running for county (i.e. Middlesex) but we don't know how the process works. Does anyone have any advice on how we can get coaches and run in competitions?