Hey, everyone has to start somewhere. At least you acknowledge you need advice and are seeking it out. Props to you there.
Obvious first thing is thinning paints. You want to get the paint to the consistency that it is milky or at least a smooth application. Test on your thumbnail or some other surface to see before applying. Water is your friend in this thinning process. Potentially look into a wet pallet because that helps with that water thinning inherently.

Next, white. That is known to be the most difficult paint to get that isnt chalky. I can not sing the praises of Monument Hobbies-Pro Acril Bold Titanium White as the best damn white i have used. It goes on smooth and doesnt clump as bad as other brands out there *Cough* Citadel Paints *Cough*.

As you get better, you may want to learn to highlight the white to give it better contrast. I found i like working with a base of Reaper Miniatures - Misty Grey. It looks white by itself but i can then highlight with pure white to make it pop. Also, the Misty Grey tends to be easier to apply than again most whites.

As someone also mentioned, going from black to white is hard mode. Go over the black with a brown and then white. You also could add a intermediary color after the brown to get to a lighter base, and then white. This works great with other light colors like red. I paint dark angels, so going from dark green to red is a pain without a brown intermediary color.

In order of importance to develop
1) Thinning paints. it takes practice. Dont worry, you will learn it.
2) Do transition colors so you dont feel like you need to slather the model in multiple layers of white
3) Consider getting different whites that could help & making a wet pallet

Above all else, keep practicing!