Start by differentiating the function. Keep in mind that a positive first derivative tells us that the function is increasing at a point, and that a negative derivative tells us that the function is decreasing.
Thus, you need to differentiate, find critical values (Values that cause the derivative to be undefined or 0, generally where a function changes from increasing to decreasing). Once you do this, pick test values from both sides of each critical value, if the derivative is + in an interval, the function is increasing in that interval. Likewise for a - derivative.
I hope this video helps, I used this guy's resources quite a bit.