1. is regular tap water alright?
Yep. If you are taking a lot of water, or several enemas you may want to add some salt or baking soda to it. Some use a mild soap solution (Dr. Bronners was mentioned above, good choice_
2.I'm supposed to use Warm water right?
Or cool water. Whatever is most comfortable for you. Just don't go too hot. Keep it under about 105 F. (I have heard some say 110, but to me that seems a bit hot)
3. what position should I take?
whatever works for you. Some like it on their back, some in knee-chest, some in the 'sims' position (left side, right leg drawn up). I like mine standing.
4.Should I hold it?
If you can. And, it depends on why you are taking enemas - health or pleasure.
5. What's the best way to get from enema position to the toilet without a MESS?
Carefully! Seriously - move slowly, and not when cramps are happening.
6. How high should I hold/hang the bag
Most say somewhere between 12 and 15 inches above the point of insertion. Good place to start. I like mine a bit higher than that. Sometimes I'll hang it 36 to 48 inches above. YMMV.
7. How often is too often?
Depends on the person. I have heard a lot of people say no more than twice a week. And I have heard a lot say that every day is fine.
8. I have heard about weakening in the anal muscle, how is this prevented?
I don't think this is really a problem unless you get really carried away.
Research, research, research. And Read, Read, Read.