So, after 10 years of playing DND with the same party, a buddy of mine is DMing his first adventure. So I thought I'd try something out that i haven't done before, playing 2 characters.

See, the idea that I had was to play a Bard/Rogue, but since i figured multiclassing at lvl 3 might be a bit dull decided I'd try playing siblings. Olo, a Halfling Rouge with a fascination of music and his brother Odo who was cursed to be a mimic (who can only turn into a small thing once a day) a year before meeting our new party. I talked to my bud and he said he was cool with the idea.

I've been reading up on Mimics as PCs and nothing has seemed to help. Some homebrews were cool, but it all seemed a bit overpowered.

This one was probably my favorite.

I'm not too sure about the stats though.
how should I go about this to fairly play this character

Most sensible thing to do is to talk about the idea with your GM. Not just get their permission, but show them the sort of stuff you found and see what they think.

If someone was going to do this in my campaign, yea, it's kind of OP. their racial benifits alone equate to to 3-4 full feats, which is huge. You essentially get the bar fighting and grappling feats for free, some shape changing/stuff and you still get 3 stat points. If I recall, most races are balanced with the idea that their racial traits are equal to 2.5 feats, or 5 stat points. So you are correct that the mimic is on the OP side.

I'd think about exactly why you want to play a mimic and tailor your racial traits towards those. If you like the idea of tricking people by changing into things, maybe give your mimic an ability that can only be used after a short or long rest. one based on a low level illusion spell like disguise self. you can always pick up the grappling feats as you level if you want them. You can do the reverse if you're more interested in being a bity boy, with grappling bonuses being built into the mimic and then taking a perk like Magic Initiate to pull off the magical illusions.