I know these are basic questions, but I know nothing about painting!! I want to update my house a bit. The walls are painted a basic white color and I want to paint over them. With existing paint already there, do I need a primer to paint over it? Or can I just use paint right over it?
Secondly, the wood trim where the wall meets the floor is its natural color. I'm pretty sure it's been sealed over with something. I want to paint these white. Do I have to sand them down, prime them and then paint them or is it ok to just paint it over in primer and then paint?
If your walls are in good shape and you know they have been painted with a latex product, and you plan to paint with latex, you do not need to prime them unless you are making a dramatic color change. Then, a primer, tinted to 60 percent of the paint formula will make your project look a lot nicer. You do want two finish coats of a nice quality paint store brand---not a cheap box store brand---paint on the walls after.
Make sure your walls and ceilings are clean before you start. I call in cleaning crews for my projects. They are not that expensive and they come with an army of appropriate size and with all the tools, toys and materials to get the surfaces spotless in a manner of hours.
As for the trim, if you are sure you want to paint over nice natural wood? You need to degloss the surface somehow so primer and paint will stick. Sandpaper works well. Ideally, since it more than likely has an oil-based varnish or poly on it you will need to prime with an oil-based alkyd primer before you can expect latex paint products to stick to it for long. Then paint over it with a nice paint of your choice.