i want it to steal heart scales from luvdisc. Like i got a mightyena but it learns thief at lvl 1 and i dont think you can get mightyena at lvl1 right? i am on my way to 4 th gym so i would like a method do-able at this stage of game.
You can get a Sentret in Slot 1 of
Simi's Friend Safari, which gets both Frisk (to check it the mon has a Heart Scale/item) as well as Covet (to steal the item) and Thief by TM. Mightyena can learn Thief at Level 1, which can be obtained using the Move Relearner in your Secret Base in exchange for one Heart Scale.
if you want a trick, here is something I used during Gen 7:
You need one more Pokémon, Butterfree
Frist, whit Compond Eyes, the Butterfree will double the chance of an wild Pokémon having an Item. Make the butterfly faint and let him in the first slot of your team so the effet is still in effect and put your Sentret in second place. Then, do the usual tactics.