1. Really, just add water. The consistency should be like full-fat milk, or there about. You may find you now need two coats to have smooth colour, but this is a necessary evil. Experiment, watering down paint is a dark art that will take you a while to get consistently right. If you aren't happy with the consistency, change it. Add more paint if it's too thin, more water if it's too thick.
2. Mix them together. Nothing more complex. It will dry up yes, so if you're painting a whole army, buying empty paint pots and mixing large batches that will keep might be an idea. Again, practise. Almost every colour I use when I paint is mixed, and I do it enough I can get it consistent by eye.
3. Treasure these moments. They will come more frequently with time.
4. 2-3 Hours usually. It's a sliding scale. You can spend as much time as you want to spend, up to the limits of your ability.
5. Nope, just don't let it get out of hand. You don't want to do what I do, and paint a squad of a new army, then never any more!
6. Paint only if it runs out or has dried up completely. Brushes whenever they stop forming a point, and even then they get recycled as drybrushes and other non-precision work. I use Windsor and Newton Series 7 Miniature brushes - about three times more expensive than
Gw brushes, but if cared for will last ten to twenty times longer.
http://www.winsornewton.com/products/brushes/for-water-colour--gouache/series-7-kolinsky-sable/series-7-miniatures-short-handle/
7. Brushmarks are caused by one of two things - either the paint is too thick and you're leaving behind the imprint of the bristles, in which case thin your paints. Or, what looks like brush strokes are actually the layer underneath the paint still showing through, in which case use thinned paints and use more layers. if you're painting models in bright colours, be sure to use a white undercoat.
GW also do a range of foundation paints which cover very well, but still need thinning.
8. Cut off the grip. It's there only if you use it on a model without the model actually holding it. If he is holding it, cut it off so it fits on the right hand.
9. Try here, and sort for 'beginner' articles:
http://www.coolminiornot.com/article
Honestly, they key to painting is practise, patience, neatness and asking for help!