To answer your questions, I'll start with the beginning:

1. Preparing the surface - Yes, I do prepare the surface before painting on it, whether it is canvas or wood. Even though most store-bought canvases are pre-gessoed, I like to add another coat or two of gesso before I start painting. This gives the canvas a nice "tooth", making it easier to paint on it. If you want to learn more about gesso, check out my page all about gesso, where I discuss different types of gesso and also explain how to prime a canvas with gesso.
2. Choosing a paintbrush - When I paint abstracts, the size and type of paintbrush that I use depends on how large of an area I want to cover with paint. For large spaces, I'll use a flat brush that is around 2 inches wide. For medium spaces, I might use a flat brush that is about 1 inch wide, or use a similar sized bright or filbert brush. For small detail, I use a round or pointed round paintbrush. For teeny tiny detail, I use a very small pointed round brush. You can learn more about different brush types, and what they are best used for, on my page about artist paint brushes.
3. Painting - When I paint in abstracts, I use the paint straight from the tube or bottle. I just squirt some onto my palette, wet my paintbrush in water, then dip it in the paint and go for it! I have found that acrylics in bottles tend to be more fluid than the acrylics in tubes or jars, so they spread easily on the canvas and work well for details. If I use acrylics from a tube or a jar, I usually wet my paintbrush and then drip a bit of water onto the paint and mix it together until it gets nice and fluid, adding more water when/if necessary. If that doesn't work for you, consider making the paint more fluid by adding some Acrylic Flow Improver or Acrylic Wetting Agent (these link to Blick Art Materials, and if you make a purchase I get a small commission that helps support this site).