Good start. I would suggest maybe applying some washes to add some depth and shadow to the mini.
Yes, maybe get a smaller brush for the fine details (not sure what you have now). If your game store has Army Painter brushes, those are OK for beginning hobby painters. I like their Hobby: Precise Detail brush for small work, and they're relatively inexpensive. There are higher quality brushes out there, but for a beginning painter I wouldn't advise spending a ton of money on brushes. Practice with less expensive brushes first, then upgrade as your skills improve.
You don't *need* to water down Citadel paints, but you might want to. In fact, on the Warhammer TV channel on YouTube they do use water when painting. Example:
https://youtu.be/D0WyyiPkWVY?t=1m39s
Check out the Dakka tutorials section, and browse through the articles. Lots of great info here. Some of my favorites:
http://www.dakkadakka.com/wiki/en/Beginners_guide_to_painting
http://www.dakkadakka.com/wiki/en/Dakka_Modeling_FAQ:_Highlighting
http://www.dakkadakka.com/dakkaforum/posts/list/374651.page
Tons of inspiration in the P&M blogs section as well.
The more you paint, the more you will learn about painting. Good luck!