How do I destroy a planet once its been built?

You click on the planet's name on the Overview, and then click abandon. It will ask you for your password. If you're asking how to destroy someone else's planet, you can't.

What should I be looking for in determining if a colony is worth keeping or not?

Size. Anything smaller than about 200 fields isn't worth keeping. Temperature factors in later but I don't think you need to worry about that right now.

What are the effects of having negative energy use?

It decreases your resource output. You'll want to stay out of the red.

Thanks alot your being very helpful,
What do symbols next to names on the galaxy screen mean?
For example (*)?

It's my job. If there's a star, that means there's been activity on the planet during the last ten minutes. Either the player, a probe, or an attack. After 10 minutes, you begin to see numbers, 11-59, signifying how long it's been since the activity. Of course some players think it's clever to add (*) to their planet's name to dissuade attacks but you can see that in an ESP Report.

Can solar satellites be destroyed if you get attacked?

Yes