A lot of it depends on what kind of army you want, and whether you want to have a really nice looking army or one that looks good to play with and doesn't take a day per model. Number one tip if you want to paint quickly is to use a black undercoat, you may need to give extra coats of certain colours to get them to come out, but it still ends up way faster and looking way better. Try out getting inks as well, they may be intimidating, but they're either a great way to get models looking good quickly, or an important step in making them look great. If you're painting quickly, particularly organic troops like Orks and Tyranids get yourself a drybrush.