Buy the best you can afford!

Oil painting is a very rewarding hobby for those who are not in a hurry. It takes several days to weeks for the paints to dry depending on color. Reds dry very slowly, browns and greens more quickly. It is best to allow the layers to dry before painting into them as your colors will meld and can look like mud if painted into each other. When beginning oil painting one thing to remember is to buy the best quality paints you can afford. For beginners, I find Winton oils to be adequate. Also, the brushes can be very expensive, American Painter makes a good brush for a reasonable price. I use a lot of "flat" brushes as they can be used to cover large areas, but can be used to create smaller details if you use the chisel point.
Since your brushes are expensive, you should take good care of them. After each use, clean with turpenoid or other odorless thinner, then use bar soap to clean any residue from the brush. Rinse the brush with luke warm water and brush the bristles across the bar soap. Using the palm of your hand, "scrub" the bristles to get the soap to lather, then rinse. Repeat this process until there is no more pigment in the lather. After the final rinse, dry the brush with a soft cloth and dip it in lard oil. Shape the brush to its original shape and lay flat to dry until your next painting session.
Generally speaking, in regard to subject manner, less is more. However when it gees to tools, buy the best. It will make all the difference in your finished project!