This is a tough question, but there are some things I would definitely recommend.
The first is figure out what you are drawin to (ha! pun intended). There are so many areas of professional illustration that require such different skill sets. Using an analogy, your basic question of "what should I learn?" is like saying "I want to take a trip, where should I go?" You need to ask the right questions to get the right answers. Specific areas of illustration are:
Editorial: magazine work. Typically very concept driven
Publishing: book work. Graphic Novels and picture books are subsets of this area. Typcially narrative and sequential
Portraiture: typically oil painting based. Very lucrative, but difficult
Concept art: entertainment design. Very hard to get into, but pays well
Storyboarding: easy way to break into the industry. Very demanding
Fine Art: fun, but very hard to break into and make a living from
So, first you need to figure out where you want to go, then figure out the best way to get there. I wouldn't say a stock answer is best here. Figure drawing is great, as is sketching in general. BUT, the dominant amount of time should be spent learning the specific area you are going in to.
BTW, I never tell people to sketch daily. There is too much mediocre work being done in the name of continually sketching. Now, sketching with focus and direction is an entirely different thing. That is how you get good, not just mindlessly drawing random things every day. Think of it this way, if you wanted to play golf, would you get good just randomly hitting a bunch of balls everyday with no criteria of success? of course not. Now if you have ever seen a pro golfer practice, they don't just hit the ball, they have a goal. "try to get the ball to land within 15 feet of the hole". That is a goal and success is much easier to measure then. Same with art. "I'm going to nail my proportions", "I want to work on getting better gestures" are examples of specific drawing goals. You can come up with ones that make sense to you.
Once you do figure that out, then do the copies and try out assignments on your own. Join groups in your area and nationally that cater to that. Go the conferences and meet people and then you are on your way.
Hope that helps some.
Maybe It was supposed to be a shoe lol