I think you'd do well to try and sort out your basic painting techniques first. Here are a few suggestions. These are aimed at helping you produce good quality tabletop miniatures. If/when you decide to enter competitions, you will find these may not cut it.
1. Thin your paint down a little. I'd suggest 2 parts paint to 1 part water. You'll need 2 coats of this to go on smoothly over a white or grey undercoat, and for particularly transparant colours like white and yellow possibly a third.
2. To avoid a silly number of layers over a black undercoat, mix the colour your want to end up with 1:1 with white and paint that as your first coat, then put two coats of the colour you actually want. This will give you back the coverage you need with thinned paint.
3. Focus on painting inside the lines. Below is a photo from the original Citadel "How to Paint Citadel Miniatures" book. This is no actual highlighting or shading (the photography implies a bit on the gold but ignore it), it's just neat, smooth basecoats.

Above photo (c) Games Workshop, used without permission
Once you have mastered Step 3, you can look at two techniques - drybrushing and washing - to add some highlights and shadows.
For drybrushing, the Citadel "How To Paint Citadel Tanks" has probably the best drybrushing article I've seen in a commerical book recently (visit your FLGS and have a read of it).
As for washes I'd suggest looking on the internet for an article or two, although look for the ones that show you have to use the pre-mixed washes as opposed to making them up yourself to begin with.
Putting it all together? I wrote a basic article a while back entitled "
Speedpainting with Washes" which shows the results of a these simple techniques. You can find that by clicking the link above.
Hope this helps you out.