Having worked with students for years on courses from algebra through calculus and further, I would say that starting from a calculus textbook will not be helpful to you (and if you didn't like Stewart's book, you won't like too many others...).
At a minimum, I'd say you need to have high-school algebra and geometry in hand (which includes being comfortable with even just basic arithmetic -- I've seen many students in trouble because they didn't yet understand how to add and subtract fractions). A lot of the work involved in developing the methods of calculus are built on those of algebra and geometry (including analytic geometry and graphing). If you don't think you are going to be involved with work that uses trigonometry, you can put that off for the time being (there are courses in calculus offered -- where I work, for instance -- that do not touch on trigonometric functions).
Proficiency in those subjects is important because you will encounter many discussions (and worked-out problem solutions) which will assume that you are familiar with various topics in algebra, geometry, and sometimes even things like basic number theory (what are prime numbers? how do you factor a number? and so forth).
It will also be important to become comfortable with applications problems (what folks usually call "word problems"). A major problem for many students is that they do not get enough experience with how to turn the language of a stated situational problem into the language of a mathematical equation or a stretch of analytical calculation, so that they can then use the techniques of the math they've been learning. (This seems to be something that U.S. schools, at least, have become rather slack about...)
You should also be prepared to spend a good amount of time practicing a variety of problems. I tell students that learning to work successfully with mathematics is similar to learning to become fluent in another language, and requires comparable amounts of time. As you come across even "worked-out" problems, you should make sure that you can follow the discussion of the solution and fill in any details that are omitted (these will usually be "obvious" details, but not always).
It will also be important for you to be able to be in contact (face-to-face, online, or however) with people who already know the subject well and have good communications skills. [I think any number of us have had to deal with those who "know their stuff" but can't get it across to anyone else...] You should of course make as much of an effort on your own to solve problems, but when you get stuck after a few tries, it is generally a good idea to get someone else to look at what you've been doing. Bogging down on one stubborn problem often leads students to start down the road to procrastination, since gratification can usually be found doing something else, instead of "that stupid math stuff". (As I've put it, it is not your life's work just to master Problem 37...)
I guess this ran a bit long, but I think this covers most of what I tell students and tutors concerning how to get better at math, or helping others with that. Like any number of other human skills, practically anyone can master quite a lot of it, but it is a lot of work and by no means a short road (there are quite a few people on this site who have spent a huge piece of their lives at it!).
P.S. Online videos and such can also be quite helpful, but no one learns much just by watching -- there is no easy substitute for your own practice (sorry!).
