If you are pulling away up a steep hill you need to pull away with quite a lot of throttle so that your foot is close to the floor, then when you have reached a fairly high rev, depress the clutch and change to second gear as fast as you reasonably can, releasing the clutch again with a lot of throttle. And so on.
If you are going up a steep hill, if you try to change up a gear at too low a rev and too low a speed for the gear change, you risk coming to a halt and rolling backwards, and quite probably stalling. Basically, the steeper the hill you are going up, the more throttle you need and the higher your revs need to be before you change up. And the gear change needs to take place fairly quickly to maintain forward momentum.