Prepping before you paint can help the process go well. Acrylic paints dry fast so you have to work quickly. So here are a few tips to help.
1. Color scheme. Think about what colors you want to use for your painting and have similar shades that aren't too far apart shade wise. You don't want a big change within shades because it won't look realistic unless of course you are doing an abstract.
2. Get your pallet ready. Once you have your color scheme mix your colors with white or black to get your different shades. This will save you time when painting because you have them there and you can mix more variations of colors with the ones you have. Another good point is to make sure there is a balance within the shades. You don't want them to be so far off from each other that it sticks out and you can see this if you are mixing your colors before you paint.
3. Organize your pallet. You can organize your pallet by creating sections for instances, if you're going to do a sky mix all the colors you're going to use for your sky and clouds and place them on your pallet in a section. and a section for trees, mountains etc. This will again help in seeing your color scheme and you can always go back and lighten it or darken it to your preference. Have fun and I hope you enjoy.