you should start by evolving the E by giving you an ad bonus on your next basic attack and the pressure you can exert against melee props, always giving you some hits and abusing the range of your E to give a reasonable damage that will help you a lot in the route phase (2 hits + post shock + skin = about 150 lvl damage).
Your level 2 is too strong to change initials, try to pull the lane as fast as possible and go up on lvl 2, get close and use the E and then it will be very easy to use your Q make it easier to hit Q, always try to get close to your birthday with E and then use Q).
Its power to engage in the route phase is very large mainly in lvl 2,3 and 6, in lvl 3 you will be able to abuse your flashlight, being able to bring the ally that clicks on the flashlight next to you (can be used with flash also ).
Level 6 of thresh is very strong, you have a great exchange potential because of the damage of your ult that helps a lot in the exchanges and by your combo (ult + sentence + skinning) that can force the enemy to collide with your ult your ult can also be used to give disengage because it contains a very strong slow).
You should always put pressure on the lane when you have your Q, and with your E being able to make quick swaps, do not be aggressive without having a vision of where the enemy jungler is and always try to use your flashlight to reposition an ally (but never to collect the souls).

Thresh is a very versatile champion, can either start fights or give a helping hand to the ADC or some ally, usually you will not start a Team Fight, so your main job will be to protect and use many skills (very easy with the 45% cdr), always on the side of the carry you must use all your skills to stop who comes to threaten your life.

Q (Sentence): This skill has a very large initiation potential, can cancel jumps and other types of abilities (And of the lioness, And of the khazix, W of the tristana and even in plants of the jungle and among others)
W (flashlight): A shielding skill that takes you to the ally you click on it, can also be used to give the Teleporter invoker spell.
E (skin): almost the same as your Q (can cancel skills among others)
R (The box): Has very strong Slow that can be used as disengage, besides containing considerable damage based on your AP.

Thresh is a very good champ so you can protect your adc from assassins, use the Guardian rune for better protection of your carry, and the summoner spell Exhaustion to lessen the burst damage your carry may suffer.

-Thresh is a champion with little mobility so do not get in a cold.
-Learn to use the Hitbox of your Q.
- Abuse the flash + skill mechanics (E or W most of the time).
-Learn to use your Q to get out of trouble using it in the minions or jungle monsters).
- Do not make the mistake of getting parked on the lane, thresh has a very strong gank potential after closing the mobility boot.