First learn to write clearly and concisely. In order of importance, technical writing should be:
If you screw up #1, you lose your audience.
You also need to do an audience analysis. Writing for a seasoned developer is much different that for a beginning developer.
Technical documents are typically written in the reverse order in which users read them. First you write the reference section, then any guide/how-to, and finally a tutorial.
If at all possible, find a good editor to look over your work. They'll concentrate on the writing part.
Don't be afraid to use a conversational tone. Read your passages aloud. You will often discover stilted or obtuse phrases that need to be revised. Use the active voice. That means avoiding statements like "After the file is loaded, ..." I personally dislike the future tense I often see in technical docs, like "This method will ...". Will when? Sometime before the sun implodes into a red star?
You might consider a night/weekend class at a local community college. As I used to say when I was documenting the Unix Programmer's Manual, the first 10 years are the hardest, LOL.