Hey ToxicThunder,
Glad you are trying your hand at Thresh! He is one of my favorite champions. A lot of newer players tend to shy away from him because of the complexity of his kit, even though it can be extremely rewarding, so I'm glad you are taking the time to learn him!
In regards to gold generation, the "Bandit" mastery in the "Cunning" tree is basically a necessity on any support. I personaly like to start Relic Shield
as my starting item on Thresh for a couple reasons. The early flat health makes me a tad more durable for early engages/skirmishes. Thresh has a very powerful level 2 all in when coordinated appropriately with your ADC. Getting an early lead from a good fight can snowball your lane and allow you to provide lane pressure, essentially scaring your opponent to not even want to fight. From there, it gives you the ability to zone. I also like Relic Shield because of the Targon stack sustain. Thresh doesn't have any healing or spammable shields in his kit to help sustain his lane partner. The Targon stacks provide a means to help your ADC stain in lane longer by healing them a bit every so often. This does take practice and communicate though because Thresh does not benefit from the "Execute" on the item since he is classified as a ranged champion, which can make last hitting for the stacks frustrating at times. Be sure to let your ADC know your intentions and that they still get the kill gold when you use a Targon stack so that they don't flame you for stealing CS.
Ancient coin
is also a viable option that other people like on Thresh. This is a nice starting item when you a still trying to get accustomed to Thresh's overall kit. It increases your base mana regen in case you habitually try spamming your abilities, but it also allows you to not have to worry about CS-ing so you can focus on trades and positioning instead.
As to "being at the mercy of a random ADC", there really isn't a "fix-all solution". Definitely always do your best. Observe or even ask what their play habits are. Would they rather you play aggressive with flays and hooks for kills? Are they more concerned about merely farming and would rather you just trade and provide zone control? Or are they constantly pushing and would like you to play back line in lantern range? These things can always change game to game and are mostly reliant on your ADC. And sometimes you are paired with a bad ADC. Again, just do your best and use discretion.
Roaming is also very strong on Thresh. You can use it to escape your lane if it's going badly, or if you snowballed bot lane, it allows you to help snowball another lane.
Thresh is a playmaker; so make plays! :)
If you have any questions or would just like to learn more or just have fun playing matches, just friend and message me or comment here.
Respectfully,
Quannari