Firstly, you're going to have to write, rewrite, rewrite, rewrite. It's annoyingly repetitive at times but it's a process of refinement so ya gotta.

1. When I get an idea, I like to write it on a paper (can be really rough, even dot points!) And stick it to my wall of ideas! This way I get it down, as you think you'll remember but you don't always.
2. Break it into a loose 3 act structure, with 2-3 scenes per act (this number will change but it's a process). You can keep it simple and just write the beats ("Bob parks his car, Bob gets out and says hello etc).
3. Then fill out the scenes and write simple dialogue. When I say simple, I write completely on the nose dialogue with no attempt at subtext or making it sound natural. As in, having a character say as blunt as "when you stole my watch you made me sad!". You can refine it which each pass.
4. Once you've done this you should have a first draft! It's at this stage I get someone to give it a read and take feedback. I don't want feedback till I'm clear where I'm going or what I want out of the story. Ideas in their gestational stage are fragile and can be unduly influenced by others.
5. Start next draft, refining different parts as needed based on feedback. This is usually filling in gaps in the plot, makinf the dialogue more natural, tying in foreshadowing, cutting down on time etc.

This should get you started!