Calculus.
The teacher makes a huge difference in how easy the course will be for you. If you don't understand the way the teacher explains it, find someone or something (book, endless youtube videos) that DO explain it so that you can understand it. You can always feel free to ask someone here to explain a concept or two; generally you'll get a decent balance of idiots/trolls/nefs to actual good help.
By far, the MOST important part of taking calculus: practice, practice, practice. There are some concepts in calculus that are incredibly simple to explain, but that take a shitload of practice to master. For example, the chain rule. I can teach that rule, explain that rule, and show how it's derived (cause I never pull rules from thin air) and I can give 3 or 4 solid examples in under 10 minutes. And, then I'm done for that day. But (since I teach 5 days a week) I give homework problems on it for 3 consecutive nights to make sure everyone has mastered it. Some master it within 15 practice problems, others take 50 or more before they can apply that rule without errors. Once you've mastered that rule, an additional 25 problems takes you 10 minutes, so no real harm in loading up the homework. But seriously, I can't count how many people say "but it seemed so easy in class" after bombing a test because they didn't practice it.
Differentiation: Take the exponent and put it in front of the variable, then subtract 1 from the exponent.
Example: f(x) = x^2
f'(x)= 2x
Integration: Do the opposite and add C at the end.
There, that's everything you need to know about calculus. I charge $200/character. PM me for payment information.
So, according to schneiderguy, f(x)=e^x. So, f'(x)=xe^(x-1)?? FAIL! I don't think the OP should have to pay for errors. Or does schneiderguy mean when the exponent is a real number? f(x)=pi^4
According to schneiderguy, the derivative would be 4pi^3. Wrong again.
Anyway, OP, calculus as someone said above is like a new toolbox for dealing with real world problems. (Tis a shame that most texts/profs don't give enough time to real world examples.) The first half of calculus deals with instantaneous rates of change. Do things in the real world change? Of course. At what rate? Well, if you graph it and the graph is a straight line, then you've probably learned long ago that y=mx+b, the rate of change is the slope which is the value m. But, if it's a non-linear relationship, then to find the rate of change (which varies), then you need calculus.
Where are you? That's your position. If your position is changing, we call the rate that your position changes your speed. Or, more appropriately, your velocity (which will take into account whether you're moving forward or backward.) If your speed isn't constant, the rate of change of speed is called your acceleration. ("Deceleration" is generally treated as a negative acceleration.) There are a lot of applications in physics, but there are also a lot of other areas where calculus can be applied, such as business, biology, (heck, probably any science), etc.