First thing you should do is undo the bolt that's holding the shift cable until it's completely loose. Then adjust the H screw until it sits perfectly on your highest gear.
Once you have that, then you should screw in the barrel adjuster on your handle bars all the way to the right.
Next, reattach the shift cable bolt to hold the cable tension; you want to hold the cable taught when you're screwing it in. Not too tight, but not too loose.
From there try to shift up - if there's not enough tension / it's not shifting, just twist the barrel adjuster to the left. This will tighten the cable to allow the derailleur to shift properly.
Once it is shifting up and down consistently, it will most likely undershoot or overshoot the lowest gear. Adjust the "L" screw until it sits in the lowest gear. You also want to check that it's not going to over shift into the spokes of your rim, so it's safer to go closer to inwards than outwards. (Your bike has a protector for that, so I don't think it is possible to shift into your spokes).
Lastly, you might get some inconsistent shifting somewhere in the middle of the cassette. Shift to the lowest gear, and make sure that between the derailleur hanger cog and the gear that there's about a pinky length between them touching. You don't want it too far apart and you don't want it to touch the cassette.
If you're still getting inconsistent shifting, try small increment / decrement of barrel adjuster tension.
If that doesn't work, you either have a bent derailleur hanger or the incorrect derailleur for your cassette. You should be able to Google this information pretty easily based on the model of your derailleur.
If the derailleur came on your bike new, you can basically phase out the possibility of it being the wrong derailleur.