How many ways can you build a planet? It depends on what you're trying to do.
If you're building a panet for a backdrop to a story line, then you don't always have to build the entire planet; all you need are the parts of the planet which have a direct bearing on your story.
For example, if your story takes place entirely in a specific valley, then all you need are the facets of the planet which directly affect the valley. The typical desert clime on the planet isn't necessary, nor the mountain atmosphere.
Another way to build your planet is to build the planet first, and see what story ideas can be found wandering around your freshly-minted earth. This is the approach we'll be taking.
The best way to begin is to focus on one extraordinary feature. Providing too many extreme features makes the planet complicated, and confusing. By focusing on one or two strange twists and what can cause them, the rest of the planetary features can be easily arrived at. It's useful to make those features visually dramatic, as well. The added "strangeness" brought to the world by such features increases the interest in the world.
Once you've selected the "special" features of your world, the rest follows along. For example, if you want a world that's rich in diverse ecosystems, you might start with an earth-type planet, but slow it down and increase it's axial tilt.
Libraries and the Internet are good places for research on those portions of the world you're weakest at. If you have a specific question, you can resort to them for specific answers.
If you have a more general question, perhaps a good place to start is with a local university or scientific institution. When you speak with someone who is a relative expert in the field, you can often get the feel quickly for whether what you're trying to do is even possible.
And if it isn't, don't get discouraged. It's the exceptions that provide the interest and the challenge in this. If the effect you're striving for doesn't work the way you wanted it to work, keep trying. Find a way to make it work, anyway. The result is sure to be interesting.
What happens if you make a mistake? Don't worry that much about it. It's important to get the crucial pieces right, but when dealing with some of the less important details, a mistake isn't as bad. Writing is about simultaneously telling the truth and lying; you need to tell the truth about the small things (the fundamentals of the scene) in order to make the larger lie you're telling (the story) acceptable. If you're dealing with a story taking place in the jungle on your planet, a small mistake in orbital mechanics isn't as bothersome.
Some suggestions for the primary physical characteristic of the planet:
- Extremely diverse ecological niches
- Big Volcano which erupts periodically to create a mini-ice-age.
- No large bodies of water
- Very slow rotation
- Earth-class planet is really a moon of a gas giant
- Tide-locked moon of a gas giant
- Widely eccentric orbit
- A single large continent
- Large bright "moon" (or gas giant) lighting the night