Re:
- My one desire is this: to live a good Christian life.
That is perfect. Do not capitalize the first word of a phrase following a colon.
Re:
- Be sure to take the following items when you go camping: a torch, matches, and a first aid kit.
That is exactly right.
Re:
- There is only one other possibility-to travel by air.
That should be a dash instead of a hyphen.
Re:
- He asked me where the library was?
That's not a question but a statement. Replace the QM with a period.
Re:
- To do satisfactory work, is my aim.
The comma is there for no good reason. Delete it. (Never put a comma between subject and verb.)