I hear two common problems with this word.

First of all, the “b” is silent, so don’t pronounce it! It rhymes with “out.”

Secondly, it can be both a verb and a noun. As a verb, it is regular.
Simple present: “I doubt that his story is true.”
Simple past: “She doubted her knowledge.”
Past participle: “They have doubted many things in the past.” 

As a noun, it is non-count. “I have doubts about this.” Think of it like  pants – you would never say “I have a hole in my pant,” would you?

“Teacher, I have a doubt” is a common phrase, which shows an influence from a student’s first language. However, in English, it sounds awkward. Say “Teacher, I am not sure about this.” instead.

Now you have no doubts. Good luck!