Radaway is rare and expensive, so it’s best to use it only if you have no other option. The two most effective ways to rid yourself of rads are with the Radaway medicine and with a doctor. In this game, rads eat away at your health bar and lower your total HPs until you cure yourself.
My first tip here is to just accept that this is a fact of life and don’t stress too much about it. Radiation poisoning is a real problem at the beginning of Fallout 4.