It had a bunch of different phases for me, as my tolerance to THC developed and changed. The first time I smoked I didn't really notice much. My friend who later smoked for the first time said it felt to him like a really tame alcohol buzz (which it basically is... THC is an alcohol and it similarly fills your blood-stream).
The first time I really got high, however, I had fun. I'm normally an extremely coordinated person with a well-defined sense of balance, but I would have trouble placing steps. The first time I smoked too much, I was pretty much unable to comprehend movement, I was scared to drive myself anywhere and even though I could manage to walk it was with much difficulty. I was quiet and introverted, unable to participate in my friends fast-paced semantics because I couldn't get my thoughts into speech.
As I smoked subsequently, movement and speech became easier. Anxiousness subsided into a general calm, comfort, even amusement. My mind would race a mile-a-minute, however. I recall once pacing around for a solid half an hour before going to bed, just answering questions in my head. Stupid shit, deep shit, whatever.
Ultimately, you just get to a point where you tell the difference between being high and not. You feel normal when not high, and then you feel slightly different when you are. Like you've got this secret that nobody knows, a private joke between you and yourself. It can either make you anxious in large groups of people (because you think everyone cares enough to notice something is wrong, or is judging you or whatever), or it can be downright hilarious. Depends on your personality.
Ultimately: don't get carried away with it. It's a great occasional time-killer, but it's best to avoid if you've got anything else you want to get done. I didn't even try it until I was 20 years old, because I have always believed that the mind is still maturing through your late teenage years and I didn't want to introduce any chemicals that may alter that.