Apparently I can't use a pet who is a higher level than my account level, so I'm just wondering how do I level up my account?

level up a pet who is the same level then your acount will raise your acount level as well. Simply train your team and you will eventualy be able to control the pet you wanted to control.

Adding onto what Cian said:

Your account level is determined by the highest level of a pet that you have trained yourself. For example, if you train your starter from level 0 to 50 and your starter is your highest level pet that you've trained (not bought) then your account level is 50. You cannot, though, skip levels. If you're level 50 and you buy a pet that's level 61 and train that pet to 62, your account level will still be 50.

what if, for example, someone is level 40, then they buy a pet for 30 and level it up to 41, will the account level change?

Yes. To 41.

Link: https://www.sylestia.com/forums/?thread=61026&page=1#3 Author: Sakina Time Posted: 5/9/2018 at 6:17 PM Adding onto what Cian said: Your account level is determined by the highest level of a pet that you have trained yourself. For example, if you train your starter from level 0 to 50 and your starter is your highest level pet that you've trained (not bought) then your account level is 50. You cannot, though, skip levels. If you're level 50 and you buy a pet that's level 61 and train that pet to 62, your account level will still be 50.

??? What.
The user level solely depends on the highest level that a pet has grown to while in your account.
You buying a pet at a high level and raising it to the next with like, Missions/exp orbs, definitely counts to raise your user level to that level? Or at least it did last I knew.

Backing up what Scath said. I've seen people's levels jump up from lower levels due to another higher-leveled pet of theirs leveling up after a mission or exp orb.

Thanks everybody! I understand now :)