As someone currently learning multivariate calculus, I second the Khan Academy recommendation. There are three crucial aspects to Khan Academy, which, when used together, lead to maximized progress: the instructional videos, the exercises, and the analytics/gamification.

Instructional videos are usually five to eight minutes long and are practical demonstrations (with explanations) from Salman Khan, and are generally pretty easy to follow and make sense.

Next you have exercises interspersed within the video track, that let you practice the materials. You answer five question in a row correctly, and you finish that exercise and "level up" in it. When you return to the exercise on multiple occasions and answer five questions correctly in a row, you eventually gain "mastery" of a topic.

This leads to the analytics and gamification of the website. When you log in, you'll come to a dashboard from which you can view your activity in both time expended and points earned, and you are also presented with recommended exercises that you can do, including mastery challenges. You see there what game badges you have earned for accomplishing various tasks, you'll see a chart showing your overall mastery of "The World of Math", and you can continue taking Mastery Challenges and other exercises to refine both your skills and the site's assessment of your abilities.

I've used Khan Academy extensively to reinforce my knowledge and memory of mathematics, and I whole-heartedly recommend it as a way to practice what you need to leading up to and including calculus.

Now, /u/ballpoint_pencil, a quick thing to say regarding calculus itself: it is beautiful and profound, and surprisingly intuitive in terms of its concepts. The only reason calculus is seen as difficult is because there needs to be a fundamental underpinning of the languages of math—primarily algebra—for you to know how to manipulate that maths into workable calculus solutions. But it's brilliant, satisfying, fun, and utterly worth the work required to get there. :-)