I think every rider should wear the safety pads, however they are most important for downhill and freeride disciplines as during those slides it’s really easy to crash and injure your joints. Nevertheless, if you’re a beginner you should wear them both for the safety and for the safe feeling you’ll have. Here are a few good choices:
Triple 8 KP 22 Knee Pads (~$25) These kneepads are really heavy duty so even in hard falls they should save your knees.
Pro-Tec Street Knee and Elbow Pad Set (~$45) This set from a solid brand PRO-TEC includes elbow pads as well. Get this set and together with a proper helmet and slide gloves you’ll feel really confident learning those slides.