Which hand should I use for signing?

As you might have noticed, one hand is more active than the other one. The more active one is usually the hand you also write with. Depending on whether you are left- or right-handed, you use your preferred hand to sign or at least execute the active movements.

This applies to fingerspelling as well. For example, as a left-handed person, you use your left hand to fingerspell words, and therefore your left hand is your dominant hand.

Everyone can decide for themselves whether to use their right or left hand as the dominant hand. The other hand is used more passively. Just keep in mind that you shouldn't switch your dominant hand. If your right hand is your dominant hand you should stick to that. Otherwise, it may confuse your communication partners.