Conplanet/Climate
Creating a reasonable and internally consistent Con-climate for your world is both a very challenging and very rewarding experience which makes possible great realism when using this planet as a backdrop for a story. Knowing the climate can also help with forming a culture for the people who live there.
Before you start working on how weather moves on your planet, however, it's a good idea to map out your world and get an idea of major geographical features (for example, mountains and large bodies of water), for these have an effect on the weather, and thus the climate, of your world.
Some useful tools, once you've gotten everything marked out and are ready to start mapping weather, are:
editWeatherbase is a fantastic tool which contains information on the weather of regions around the world (real Earth). If you aren't opposed to taking a cue from real-world situations (and your planet is similar enough to Earth), then you can select a city in a region which is similar to your own, and get a good idea of what goes on there.