What is wrong with a/c in my car?

[stevie_k_29]

My car feels like the engine or the steering is vibrating on and off while I am at a standstill. During the time that it is vibrating my a/c no longer blows cold air. I have also noticed that the air is not as cold anymore even when it is not vibrating. I drive a 1999 Volkswagon Beetle. I am a single woman who knows the basics about cars but not a whole lot about a/c stuff. I don't want to be taken advantage of at the dealership b/c I don't know what they are talking about. I would greatly appreciate a little insight before I take it to the dealership. If anyone knows what might possibly be wrong, what it will take to fix it and how much the cost range will be, you would be my hero! Thanks.