Search the posts. It's a very common question with a lot of answers. I learned by finding all the a' s on each string (to the 12th fret ). Then b,s and so on. Then play them up and down on each string to a beat. Up and down on a, then b and so on. Look for prompts and patterns that make sense to you. There's heaps of tips and some good YouTube ideas too but ultimately it's a commitment that you have to make. 15 minutes a day religiously and it will suddenly dawn on you and your playing will change forever. There's a fretboard trainer on this site that can help a bit but not as good as guitar in hand. Hopefully you know the octave patterns. If not then I think that is required before filling in the gaps. Hang in there mate. I thought it was impossible, and then suddenly.......