1. Shift onto your smallest cog - then feel the shift cable. In this shift position the cable should feel taut but not tight. It's not too uncommon for the shift cable to be so loose that it's a whole gear off. If that's the case, disconnect it from the derailleur and reattach it so it's taut.

2. Shift just 1 gear and see if it shifts up. If it doesn't, tighten the cable with a barrel adjuster 1/4 turn. Try it again and tighten again if necessary. Once the indexing is right for the 1 to 2 shift, it will usually shift the remaining gears one-at-a-time crisply.

3. If it doesn't shift across the whole cassette crisply, your derailleur hanger is probably misaligned. You're probably going to have to take it to a shop with a gauge to fix that.