Hi and welcome to this part of the forums !!
You're starting from scratch, so I'd recommend reading your way through the posts here for the last while - good way of getting the basics, also recommend the FAQ.
I'll have a pop at your questions, but TBH there's oads of stuff to take into account so take my suggestions as guidelines - no doubt others will be along with alternative/additional views.
Questions:
Does anyone know of the best place to go to demo pj's?
Nexnix are forum sponsors and have a sterling reputation with members here - they have decent demo facilities. (Horsham in Sussex)
What are the best types at the moment?
Ha Ha !! Don Flameproof suits now!! Have a read of posts about the Panasonic AE300, Sanyo Z1, Sony HS10, NEC HT1000 and lots of others.... There are new projectors due to come onto the market in the next few months - see recent posts about Sanyo Z" and Sony HS20, for instance, NEC HT1100 also due, and new models from Sharp etc.
What's "best" depends on your eyes (and your budget....), go out and see some projectors in action, ask if there are forum members living in your area who'd demo their systems for you.
Would i be able to watch "normal" tv on one or only films?
You can watch TV programmes on a projector, no problem, but I wouldn't recommend a projector as a replacement for a TV - there are members who do use them as a TV, but it just isn't for me.
Does the room have to be totally black all the time?
Decent light control - full blackout imho, is key to decent picture quality from a projector. The "blackest" black you'll get depend on what light is reflected from the screen when the projector is not turned on; ambient light falling on the screen is just going to wash out the image, the degree of washout depends on the level of ambient light. You'd need to see these conditions in operation to judge the effect for yourself.
Depends on the projector - most are fairly quiet - 28 - 29db is common, "noisy" is a pretty subjective term - you need to see these in action in a typical installation to make a judgement.
My room is about 18-22 foot long byabout 12 foot wide...what size screen could i get?
I wouldn't recommend a screen width more than 8ft, above this level picture performance begins to degrade, 7ft screens are more common. Seating distance from the screen is recommended at 1.5 - 2 times screen width. There's a link to stuff on screen sizes etc. in the FAQ.
Hope this gives some pointers.
All the best,
Sean G.