How do i sober up from drinking and smoking pot?

Start with stopping: Very few people at age 18 will develop any serious withdrawal from either alcohol or marijuana, but craving, sleep problems, anxiety and other things may occur. You are on methylphenidate, I presume for ADHD, which both pot and alcohol may make worse. Talk to your health care provider about getting support for gaining sobriety - there is help available. Wanting to is the first step so congrats!