Well while going to the gym will help out a ton, diet is what is really going to make the difference. If you are actually on a strict diet, and eating properly, I'd say your friends will start to notice a difference in about 2-3 weeks. You probably won't notice much for about a month, because since you are constantly seeing yourself and its all small changes, it seems like nothing is happening. This can be discouraging, but getting bigger or stronger shouldn't be your only goal when going to the gym.
I remember back when i was constantly going like 4-5 times a week, I was getting bigger and my friends all commented on it, but I seriously felt like i looked the same. One day though, I just noticed how big I had actually gotten and it was a hell of a confidence boost
As far as your strength, you should notice improvements almost right away. You will get stronger before you get bigger. Just push yourself each week, and you should be doing weights you could never do day one after about a month.
Remember, you gotta eat big to get big. Diet is key. It's what I never got down which is why I never saw huge results. I was a whole lot stronger then what I used to be, but I didn't eat enough so I never gained alot of mass.
Hope that helps.