I have done weed once, and it was alright, but I don't do it at all really. Was only a one time thing. As for alcohol, I have had my share of good experiences, and as well as bad, but for advice on drinking, the only thing I can say is: Since this is your first time, I'd drink with people you are VERY trustworthy with, don't drink with people you hate.
You also have to be very careful when drinking. When you drink, your body becomes very intune with the alcohol. What that means is, you will start to feel really happy, almost like floating, and you will really like the sensation. This is called being, or just starting to be, buzzed, and it is a very nice feeling. On the other side, as you will like this feeling, you will want to DRINK MORE alcohol to increase the sensation. Don't do this!!! Yes, people will say "no man, it's fine. keep drinking!!!" don't. fucking. drink. more. You should stop at this stage. This is where you want to be.
On the other hand, if you don't stop and give in to your senses and drink more, you will eventually become drunk. When you are drunk, you are honest to God out of control. You may think you are fine, but reality is, if you don't feel like yourself and your speech is slurred, as well as your balance is REALLY fucked up, chances are you're drunk and will DEFINITELY need to stop drinking. You can cause a great deal of damage to yourself and everyone around you at this point, and you can very easily get alcohol poisoning and die.
So yeah, sorry if I made it seem like not a good thing to drink. If you're with good friends, it's a lot of fun, but just remember to take it very slow for your first time.