When it comes to finding the perfect tension for your bobbin, there are a few things you need to take into account. The type of thread you're using, the size of your needle, and the stitch you're trying to create all play a role in finding the right tension.
Generally speaking, most people find that a good starting point is to set their bobbin tension at 4 or 5. This gives you a nice balance between having too much tension (which can cause your stitches to be too tight and potentially break your thread) and not enough tension (which can cause your stitches to be loose and sloppy).
Of course, the best way to find out what tension works best for you is to experiment! Start by sewing a few test swatches with different tensions until you find one that gives you the results you're looking for.