Bring the shovel straight up, keeping the torso straight and letting the arms and legs do the work. Gardening shouldn't give you anything more than an aching muscle that quickly gets better. If you used both arms, the stiffness should be in both arms. If the pain is localized on one side or in the center, and lasts more than a couple of days, seek medical help. If the pain is severe, you may have damaged your back, and a hot shower might make it worse.
Ice is always good.
Listen to your body. If you do hurt your back, get to the doctor.