What do you do if you're hanging out at a friend's house and someone suggests you guys start smoking or drinking, but you're really not ready to do that? Here are some things that you can do to get out of that situation. Something I always did is just be honest and say no thank you if someone offers it to you. You don't have to make a big deal and be like, say no to drugs, but just say no. Say no thanks, I don't want any.
If you're really in a situation where you can't say no for some reason, I don't say lie a lot and honesty is definitely the best policy, but here's a place where you can totally fudge the truth. Say your parents are coming to pick you up soon. Say you have a big project to work on later and you just can't drink anything. Or, with cigarettes, you can smell that on your clothing all day long and your parents would not improve. Anything, anything that's plausible, make it up.
If your friends are smoking and drinking the honest truth is that eventually they're going to get caught. You probably don't want to be there when that's the case - especially if you aren't even drinking. If you know that that's what your friends are going to be up to maybe decline that invite. See if they want to hang out another time doing some nonalcoholic activities. If they're really your friends they'll be fine with that. Hopefully, they aren't going to try and make you do anything that you don't want to do.
Absolutely never get into a car with someone that has been drinking. Don't let them tell you that they do it all the time, or that it's totally fine, or anything like that. Don't get in the car. It's extremely important. Call your parents. Call your sister. Call someone to come and give you a ride instead. Honestly, they'll understand the situation.
Remember that you have your whole life to do this stuff. If you're not ready to do it that's completely normal. That's okay. That's preferable. You've got all this time later on to do this stuff, so just put it off for a couple of years.