Prepare to teach them the rules.
A bit like video games have levels of "tutorials" that are essentially a game exercise, you will need to set up simple situations and encounters and guide them through the rules several times before you can master them.
Make them responsible for knowing their own characters.
D & D characters have many special powers, spells and magic items with unique powers that can be used in so many different situations that must all be reconciled. As a DM, you are responsible for deciding how these things work together, but players must be able to tell you exactly what their powers and abilities are and how the work is done.
If a player asks you every time to explain what his options are, his attacks, how to calculate his attack roll, it shows that he does not know his character well enough. In this case, you may have to push them away and make them understand for themselves. Like spoiled kids, if you do it for them every time, they will never do it alone.
You do not have to be a fool to accomplish this, although you have to potentially call players at the table.
GM: Ok, Warrior, it's your turn, what do you want to do?
Warrior not prepared: J 's closest attack … I got a 6.
GM: It's a miss …
Warrior not prepared: Oh, wait, I need to add my skill bonus …
GM: Ok, what is the total then?
Warrior not prepared: Euh, I do not know … *look at you in the expectation*
GM: Ok, we escape you while you take care of it. Move on to something else … Mage! It's your turn!
Guaranteed that after being jumped several times, the unprepared player will score his attack bonus!