For normal driving, try to keep the revs between (approximately) 2,000 and 3,300. Above that, shift to the next gear, below that, shift to the previous.

Shifting at various speeds may also be helpful initially. For example, start on first, of course, but shift to second arond 15mph, from that point on, shift to the gear with the same number as the speed. 3rd gear at 30mph, 4th at 40, 5th at 50, and 6th at 60.

While driving on the highway, at higher speeds and on the top gear, expects to be at that 3,000, or even higher, but that's ok.

The grinding happens because the clutch pedal isn't being depressed fully. Focus on that, and you should be OK, as long as your clutch release technique is fine, but you didn't mention that.

I would even suggest practicing your pedal work and shifting simultaneously with the car off, then with the engine running, but parked. That way you will notice if you're grinding the gears or not. Of course, while parked you won't be releasing the clutch unless you intentionally want to move. Also, above, above 2ng gear, if you release the clutch while parked you will just stall the engine.

Hope this helps.