What to Paint?
Most beginners strive to make their paintings look as "real" as possible. From my own mistakes, I know it's hard to know which subjects will be easy and which will be hard until you have a lot of experience. For your first few paintings, you'll want to choose something that will challenge you, but be easy enough to give you a good experience and a finished product you are encouraged by.
A lot of art teachers recommend starting with a familiar scene, like the view from your window, or a self-portrait. While these are things you'll have access to, I actually don't recommend this tactic. The more familiar you are with the way something should look, the more critical you'll be of your painting of it. For the first few paintings, you'll be busy enough getting used to the materials. Just have fun. When you're ready to choose a project, consider this advice:
1) Portraits are Really Hard for Beginners
Although I'm mostly self-taught, I took a few art classes during high school and university, and nearly every class started with a self-portrait - one of which had to be life-sized. I really struggled through all of them, and didn't end up keeping any because I was too frustrated by the end of the exercise. Portraits are hard because there is very little tolerance for error. If you misjudge the position of an eye or nose by even a centimeter in your initial sketch, the portrait will look wrong, and it's hard to fix it except by starting over. You'll be even more critical of mistakes in a portrait of yourself, because your face is the one you know best. Wait until you are used to the materials first.
2) The Argument for Landscapes
I recommend landscapes as a good starting point for beginners. Landscapes are much more versatile and forgiving than portraits or still lifes. Your barn came out looking like a spaceship? Easy - just paint a hill over top of it. Spilled some coffee on part of the sky? Looks like a good spot for a tree, doesn't it? You don't have that kind of freedom with portraits. In landscapes you can still paint a fairly realistic looking scene, if realistic is what you're after, but have a lot of room for changes.
3) The Argument for Still-Life
I think still-lifes are more difficult than landscapes, but they have their own special advantages. Unlike with portraits or landscapes, you have a lot of control over the scene you are going to paint. You can control the color palette, composition, subject matter and lighting by arranging your scene before you start. If you are painting fruit, you can even go to the grocery store and select the fruit yourself.
4) Remember to Modify Your Photos
This is something that took me years to really get. Painting scenes (portraits, landscapes or still-life) from photos can be useful, but you'll soon realize that good photos do not always make good paintings. You're not a photocopier. Don't fall into the trap of trying to replicate photos. Instead, use several photos and optimize the composition before you start. If it helps, you can cut and paste from magazines or printouts, or use a photo editing program like Photoshop to experiment with composition before you begin painting.