Hitchhiking is stupid and dangerous. If you must go to a party without your mothers knowledge you should be sensible and keep safe.
No hitchhiking, limit your alcohol and if you get involved in any hanky panky - keep it safe and consensual. Lads and alcohol, plus no supervision can lead to things that you may regret later.
I would urge you to tell your mum about the party, and to ask for a lift. Even Mums were 15 once.