To be 100% honest, you probably shouldn't be doing mushrooms at all. Shrooms have the potential to seriously mess with your mind, your perceptions, and your entire sense of reality. Being all of 15, your vision of reality has not been fully formed (I know you'll disagree but I was 15 too; or ignore me if you like and find out for yourself
, so messing with it is not a good idea.
However, if you insist on going through with it, there are some things you should keep in mind:
1) Shroom potentcy differs from species to species and, even within species, from shroom to shroom. In conjuction, different people react differently to shrooms. The only way to know what a "good" dose for you is is to try them. In that vein, be cautious. If you have a scale, I would recommend doing less then 1g your first time. If not, pick a medium sized shroom and wait for the full effect. Which brings us to...
2) Speed and duration of the trip depends on many things, not the least of which is the method of ingestion and fullness of your stomach. Eating the shrooms, it will take longer for you to feel the effects and the effects will last longer then, say, brewing the shrooms in tea. Eating shrooms on an empty stomach will speed things up, both the effect and the duration. I recommend whatever method you use, have all of your friends do the same. That way the trip will be similar for all of you and you won't have one lone sheep tripping for an hour after everyone else is down. Approximate duration of the whole trip should be 6 to 8 hours, give or take.
3) Make sure you are in a good mental state before you take the shrooms. If you are feeling depressed, annoyed, isolated, insignificant, etc., the shrooms will feed off of this energy and you may very well experience a "bad trip." But remember, it's only the shrooms talking. The experience WILL end and you WILL be back to your old normal self. Which brings us to...
4) Trip buddy. Make sure there is at least one, preferably more, people in the group who are perfectly sober. I'm assuming this will be the first time for many people in the group, and no one knows how they are going to react. Have some sober people there to help anyone who starts having an unpleasant trip.
5) Again, BE CAUTIOUS. Considering this is your first time and that you are relatively young, doing less is better than doing more (but far worse then doing none till your 21). At higher doses, the disassociative effects of mushrooms can be... maddeningly profound, for lack of a better term. In my opinion, you would be doing yourself a disservice in life by rushing towards these effects. Remember: Mushrooms will still be there in 4 or 5 years, your current view of reality will not.
Anyway, sorry for the preaching. But if you want my advice you have to take my thoughts with it. If you do decide to take the shrooms please let me know how the experience is.
Mizuke