I’m too scared to tighten my yoyo because I broke my last one by doing that, how do I know when to stop tightening?
if it gets a little harder to tighten STOP. as soon as it starts to slow down when you tighten it ussually means it’s done tightning.
Just get it as far as you can by spinning it with you finger, then give it a little extra turn with your hand.
Another method is to not grip the two halves with your fingers, but to just hold each half with an open palm and tighten it. Of course you would need to hold it with your fingers to get it started.
What kind of yoyo is it? that will help a lot. Infact if you ever have a problem referance the yoyo you have a problem with and all modifications. It helps us help you. 
It is a duncan freehand, it has the same axles as some of the other duncans. I’m used to taking apart yomega’s yoyos because its smooth to take apart and to reassemble. Duncan’s yoyos give resistance when getting close to tightening it, that’s why I can’t tell if its tight enough.
With duncans you can just stop when it resist’s. if you comes apart while you play turn it 1/4 turn farther next time.