Very good advice psilo... definitely take it easy with the weed as yes it can cause shrooms to take totally unexpected turns.
And again, seriously, you have no idea what you are in for... shrooms powerfully affect not just visuals, but your impression of all body systems and body image in ways such that you really do not realize it is because of the trip but become scared something has actually physically gone wrong... they have very strong warping effects on the emotions, you can get very confused and really wonder "what have I done? what the fuck is happening" and completely forget you took a drug... thats when you can start to really loose it and the trip is wasted... even with a sitter they may not be able to convince you its just a drug no matter what they say, but at least they might keep you from calling 911 which would be a disaster. Put them away as a hidden treasure for such time as when you have an actual good friend to accompany you... that time will come soon enough, and you will be very glad you waited.
Yea, if the girl you got em from is someone you'd need to pay to get to sit with you, and you really dont know her, you could start to get all sorts of paranoid ideas about her and her motives. NOT a good idea either.
If you have to be alone, try to get some LSD, it's much more mentally stable and friendly at typical doses.
Or maybe try 25mg 2C-I (some say thats alot, but not till that dose do you start to get really major tracers and other interesting visual effects happening... and it MASSIVELY enhances music appreciation, to a nearly orgasmic level, perfect for spending a night in your room alone with tunes and your headphones on... and its almost impossible to freak out on 2CI... it creates a very Zen-like, stable, almost "beyond emotions" highly calmed mind-state... it tends to BLOCK runaway thoughts or mental verbalization completely, whereas shrooms tend to trigger it.... I suggest 25mg 2C-I for your first time trip alone in your room... pretty much a guaranteed positive experience, IMHO).