1. They handle completely different from any other paint medium. Surprise, surprise, I know. However, due to the nature of the product it completely changes how you handle it. Coming from a background of oil and acrylic paint, it was a bit of a learning curve. As I am not use to woking with something so translucent, it's a real mind bender in the beginning. I personally like having some control over my paint, but watercolor is at it's best when you let it do it's own thing. It was hard to give up that control initially, but now it's the most enjoyable part of watercoloring. It's freeing to let the paint go where it wants and to bleed in unique ways.
2. Always start light. Watercolor can be very flexible, but it's best to start with a light hand and a little amount of paint and build up vs throwing large amounts of color down. If you are confident in your ability to work with deep color and brush/water technique, go crazy. I on the other hand, will continue to use the build up method until I feel more comfortable.
3. Know your highlights first. This can be tricky as it is the reverse of what you learn when using oil or acrylic. Highlights generally come last, but in watercolor it's the first thing you think about as you want the white of the paper to show through. The best method of doing this is to plan out in your sketch which areas to avoid. You can lay down a masking product to protect these areas as you paint, or follow with a gouache. Be warned, gouache can easily become muddy, I would suggest planning ahead to avoid issues later.
4. Paper is vital. I have been using a lower grade paper from my local craft store and it shows. The paper buckles when water is applied and this can hinder your painting a great deal. Not only will you have dreaded wrinkles, but it will cause pooling which can be very troublesome while painting. I recently purchased some 300lb weight paper from an art supply store and it's night and day. The paper can take a great deal of water and it allows the paint to absorb nicely. I would suggest using 300lb or higher to reduce, if not eliminate buckling.
5. Get a palette and watercolor tubes. You can buy watercolor in a few ways. One being the dried chunks and the other being in tubes. You don't need the top of the line liquid watercolors, but I would recommend skipping the dried stuff for the tubes. The color is more vibrant and easier to work with. This is where having a palette comes into play. You can place you colors in the palette for future use, plus it gives you a place to mix colors, which in turn, keeps your colors from getting muddy. If you are on a strict budget (like most of us) you can buy a cheaper set of tube paints which have a small build-in palette. I got mine for $10. You can also purchase palettes from $10-$50, depending on your preference and how large or small you want it.
6. Keep your water clean. I'm guilt of not following this rule and it has come back to bite me in the butt a few times. Whatever you rinse your brush with is going to be the color it picks up. Hence keeping your water clean...or be prepared for a potential mess.
7. It's not oil, it's watercolor. In the beginning you might want to treat it like oil or acrylic paint. I'm here to tell you it's not going to respond like either of those. Watercolor is all about not over working the paint. It's translucent and organic, so you can't get flat color, so don't force it. You'll only over saturate the paper and muddy all your colors up.
8. Don't freak out! You can fix it. Many people think you can't fix an accident in watercolor. That's not really true. It's pretty forgiving since you can blot color off with paper towels or by adding clean water from a brush to pull up most of the color. This is why I suggest a light hand when watercoloring since you can fix this issues very easily when you start light. If it's a dark color, you can't really pull it all up, but you can dilute it.
These are a few things I have learned about watercolor. I'm still learning and these things may not make sense or work for you. Everyone does things different, however, I wanted to share my knowledge in hopes it may help you get into watercolor. It is also my hope that you share you knowledge with me. All the best!