The goal is to feel some burn towards the end of your set without being totally exhausted. If you're brand new to weightlifting, grab a light set (5-20 pounds depending on what feels comfortable) and try to do 22 bicep curls. If you get all the way to 22 without breaking a sweat, add weight. If you can't even get to 14 reps without your arms giving out, remove 5 pounds and try again. Once you find a weight where you feel a burn but you aren't compromising your form by the 22nd rep, that's your starting weight.