Dont know what to do??

Hey so I've been trying to conceive for almost 6 months now and no luck so far..I got off the mirena in may and every since then I have irregular periods lasting anywhere from 10 days to 2 1/2 weeks so I dont really know when I ovulate. Have been to a dr had blood tests done and an ultrasound and they said nothing is wrong. So my next step is opks but I have no knowledge what so ever on what to do or the best kind to get. Any advice would be greatly appreciated. I just want one last baby like yesterday.