First of all, take note of several things about your furniture. What is it made out of? Different surfaces need different paint. Not only for how it will stick and wear, but for how you apply it. For example, have you ever tried painting wicker furniture? It's not a fun job if you're using a standard paintbrush. Additionally, have you ever tried painting anything with a lot of detail work, same thing as the wicker, not easy. Does your furniture have areas that will and won't be painted, surfaces that you need to tape off and avoid or surfaces that can be removed entirely?

Next take note of what kind of condition the furniture is in. Is it well-sealed, just needing an extra coat to brighten it up? Is it wood that has been out a while and may need a paint that is also a sealant and wood protectant? What about metal? Are you noticing any rust? There are paints made specifically to keep rust from coming through. If you have a rusty area, you don't want to put a regular paint over it or the rust will just bleed through the paint. Is it some type of plastic or artificial material? These materials can be some of the hardest to get paint to stick to so you want to make sure that you choose paint specifically made to bond to those smooth surfaces.

How does the furniture get used and where? Is it in the sun all day? Do you live in a rainy area and if so is it protected or in the rain? Is it next to the pool thus standing in the sun all day and getting wet frequently. How about trees or birds dropping undesirable sticky substances on the furniture? And who uses it and how much? Are children using it to build forts or is it strictly the occasional evening being relaxed in by adults?

For anything with a detailed surface or many different levels or crevasses, the best way to go is with a spray-on paint that also fits the other needs such as the level of sun or water received, the level of contact, et cetera. A spray will ensure an even coat and fill in all the little spots whereas trying to do that with a brush will lead to multiple layers in some areas as you keep trying to reach that little corner, thus resulting in clumping and lines.
Just be sure that you practice with the sprayer on something that doesn't matter of a similar surface to make sure you have the hang of it and can get an even coat. Remember it's a lot easier to do several light coats than to sand off drips and runs to repaint.

Flat and wood surfaces will be fine with a brush, as well as furniture with surfaces that you want to avoid getting paint on.

Oil and enamel paints last the longest. They're a little more difficult to apply as they aren't water washable, but they're way more durable. If you don't need the furniture to last more than this summer, latex is a quick way to go, easier to clean up, and cheaper than oil-based paints. Rustoleum is a brand made specifically to stop rust and can be a great help on metal furniture. All in all, there are many different types of paint designed to deal with all the situations listed above, make sure you get good advice from the paint guy at your local home improvement or paint store and then enjoy!