How do I know how much to adjust the tension, how can I get it just right?
Its probably been said but it's just trial and error. Use on a indoor trainer or a quiet carpark (or the like) can reduce that error before you go on the road.
If you're struggling to unclip, they need loosened.
If you're unclipping when you're not meaning too, they need tightened.
As a starting position, I usually loosen the left clip all the way (that's the foot I always unclip with first, it could be your right, do what's natural) and have the right clip a little tighter (it's my more dominant foot, you could be left dominant!) and adjust from there.