A full explanation of defective verbs is probably too long to fit in an answer here. I recommend you take a look at the following page:
This page (and the website in general) has the best explanation of the Arabic verb system that I’ve ever seen in my life. It explains not just *how* the verbs are conjugated, but *why* they’re conjugated that way. The full site is free and goes through all of the verb types (normal, hollow, defective, assimilated, double) and all of the verb forms (I through X). If you work through it all you will be well on your way to mastering Arabic.
Other than that, you just need to practice. Fully write out the conjugations for two or three verbs of different types every day, and within a few weeks you will have the whole system memorized.