For your first time, and if the shrooms are of average quality, I would do about 2 grams. This should be enough to make you trip pretty hard, but not too intense....it will kind of let you get a good idea of what shrooms are all about.
In my experience, good shrooms can make you feel very euphoric physically and mentally, almost like an ecstacy trip, and at the same time, if you took enough of them, you can have some pretty cool/awesome visual and audio hallucinations.
Most people say that when they are on shrooms, the walls look like they are breathing.
Also, everytime ive done them, I know im starting to feel them when I started to feel giddy and giggle alot. For me, thats the "come up" and then soon you start trippin alot harder with the hallucinations and what not.