Expressing uncertainty:
* I doubt he'll be able to finish the project on time.
* There's no doubt about it, she's the best candidate.
* I have my doubts about his honesty.
Expressing disbelief:
* I doubt that story, it sounds made up.
* Don't doubt yourself, you can do this.
* I doubt he'll ever admit he was wrong.
Other uses:
* The doubt was eating away at her.
* She was overcome with doubt and uncertainty.
* He cast doubt on her claims.
The way you use "doubt" will depend on the specific meaning you want to convey in your sentence.