I’m just starting with my team and I need help. We tried making a flywheel with an 84 tooth to a 12 tooth gear but it was too slow. So any ideas? 
Your flywheel has only a ratio of 7-1 and i wouldd reccomend at least a 21-1 for a double flywheel but i am not sure on single flywheels as i havent experimented with any yet.to get to a 21-1 you need to put an 84 tooth to a 12 tooth and on that same shaft have a 36 tooth that goes to a 12 tooth on the shaft that has the wheel on it. Of course you can go higher with your ratio by switching external gears around or you can change the internal gearing to speed or turbo. It also depends what kind of flywheel (1 wheel or 2) you have so keep that in mind there are other threads that talk about this too so take a look at some of those
we have speed geared motors and a 21:1 external gearing for a double flywheel using 5 in wheels, for 4 in wheels maybe go 25:1
for a 1 wheel shooter switch to 35:1.
also make sure whatever mount/housing you build is very solid and low frictions, use locknuts keps nuts will vibrate right out
First how gar do you want to throw? Full court, half court, or close shots? If you are using a double wheel flywheel with 4 inch wheels you could get by with a 7:1 with motors geared internally to turbo. but I doubt you would get a full court shot. If you don’t want to do compound, you might try 7:1 with turbo and 5 inch wheels.
This would be a great time to teach yourself about compound gearing.
Good luck
Good construction can not be emphasized enough. We have been having an issue with losing the set screws on our collars from vibration. The closer to the flywheel, the worse it gets.
If you have the room, swapping the collar inserts out for 0.25" screws works very nicely. I recommend doing this everywhere space provides for it. Collar inserts strip easily, and you can’t always get them as tight as you’d like.
My kids got roughly half court with 7-1, internal turbo gearing, 4" wheels, and some solid construction.
kk24’s suggestion is a good one. However, your flywheel should not vibrate at all. I have included a previous post I made, modified for clarity:
While some teams have had success with the traction wheels, they cause vibration which also wastes energy. At high speeds, the rubber tire starts to flex, the wheel becomes imbalanced, and it vibrates. By zip-tying the rubber tire down, as 8059A has done, you can reduce this effect, but not remove it completely.
A better flywheel choice would (surprisingly) be an omni wheel. These are solid, and hardly vibrate at all. One of the teams I mentor built a flywheel launcher with 2 motors and an omni wheel that was quite good. It could consistently score from the opposite corner of the field, although the delay between shots was quite bad (about 2 seconds). More motors would help with this. (I’ll leave it up to you to figure out the gear ratio.)
EDIT: I have also addressed the question of flywheel construction many times before. I recommend this post.
Thank you, everyone! I’m going to go with ThunderRobotics suggestion and make a two gear flywheel with a ratio of 21-1. 