You're certainly not the only one with this question. I've just finished my A-levels and I don't really have any close friends; I was friendly with those in my classes but I don't really enjoy socializing in the same way as them so it doesn't really work outside of school.
I spoke to a therapist and they recommended I just try to keep myself occupied, even if it's just doing something simple, such as travelling to and from a university to get used of the routes I may take. I've tried to set some goals, such as going to the gym, drinking more water, and passing my driving test. I thought that was quite a good idea considering I felt like I had nothing to work towards for 3-4 months now that exams and coursework are over. Aside from that, I'll probably be watching Netflix or listening to music. I've also had it in my mind to purchase an instrument and start learning it, so I think I'll make some progress with that.
Hopefully you can put together some ideas from this — have a good summer!