Your outer non rain hiking cloths...generally your hat, shirt, and pants and maybe socks unless you wear some kind of gaiters...then treat those. You might treat the uppers of your footwear if they are fabric. Generally I would not treat any warm layers or underwear.
You can also treat your tent mesh although people have mixed results with this. I would not not bother treating the fly.
Generally woven cloth is better at defending against mosquitoes and lighter colors make it easier to see and eject ticks. Long legs and sleeves are obviously more effective.
Careful personal inspection is useful. Carry a mirror. Have a friend check less intimate parts you can't see.
btw. Don't use DEET on gear. It destroys plastics
@OldGuyot gave you a lot of good info, so there isn't a lot to add, however, if you're hiking in a tick infested area a lightweight gaiter that holds the hem of your pants inside it can also help. I've also hiked with folks who tuck the bottom of their pants into the tops of their socks, particularly if they're hiking through a brushy area.
I simply place EVERYTHING in the Permethrin solution bucket and let it soak. At least for me, there is no skin irretation or other problems at all. I keep it wet in the bucket overnight
I soak everything then let it drip dry. Then usually use the rinse cycle on the laundry machine and the rise takes out excess permethrin.
But peretrin is not a repelent. I use the DEET-free repellents now and keep DET only as a backup
totally agree with those responses but wanted to highlight that you DO NOT use on under pants/panties as it will cause rash and ruin a hiking trip....:)