Slope is the amount that y changes, divided by the amount x changes. That is, you pick two points on the line, subtract the values of y, subtract the values of x, and divide. If you do it right (and if your line is really a line), you'll get the same answer no matter which two points you pick.
In simple problems (like the ones in this section of the website), you'll be given the two points as ordered pairs. For example, you might be given (0, 3) and (2, −1). So the two values of y are 3 and −1 and the two values of x are 0 and 2. Subtracting the values of y gives you 3 − (−1) = 4. Subtracting the valuesof x gives you 0 − 2 = −2. Dividing gives you 4 / (−2) = −2, so that's your answer.
Sometimes the two values of x are the same. Then their difference is 0, which is bad because we can never divide by 0. In that case, there is no slope, and you should say so. (If you draw a line between the two points on graph paper, it will be vertical.)
It doesn't matter what order you subtract the numbers in, so long as you use the same order for y as you do for x. In our example, say, we subtracted the first pair minus the second pair for both x and y. If we had done the two different ways around, we would have gotten a wrong answer.