I know this is contrary to popular opinion but here goes.
Alcohol shouldn't effect your erections much at your age. I personally find that getting some liquor inside my system prior to sex helps me relax a lot easier while having no effect on my erection quality. In fact I actually prefer drunk sex to regular sex. In any case drinking might help you calm your nerves, it'll get you in that I don't give a damn state of mind which really helps in overcoming nervousness issues.
As for weed, well there have been times when i've smoked and its made me real horny but in most cases it makes me want to have less sex rather than more.