I'm brand new to cycling and I have an entry level Cannondale catalyst 3. I was riding my bike on a trail and when I got home I noticed my right pedal was wobbling around. I tightened it but the pedal still wobbles around. I'm thinking the crank arm thread is stripped but I don't know. the crankset and the pedals are all default that came on the bike when I bought it. I'm on a low budget so is there any way I can fix this or do I have to get a new crank set? PLEASE HELP
the pedal wobbles in the arm
That sounds like a stripped pedal thread. If the cranks are inexpensive or dubious, replacing them could be cheapest.
But in some cases a good bike mechanic can repair a stripped pedal thread by installing a helicoil or similar coiled insert - basically they tap out the hole for a yet larger size, and the wire insert fills up the space between that and the threaded pedal axle. You and your mechanic would have to make your own decision if this is appropriate in the circumstances of your particular case.
Normally modern pedal threads are specific to each side of the bike (the left pedal gets a reverse or left-hand thread) such that they will self tighten from precession when riding, especially on something with a freewheel where you cannot exert real reverse torque anyway. A pedal with the wrong thread for the riding torque of the side on which it is used will indeed unscrew, but being able to create that situation with modern components would be fairly unique to an exceptional case such as riding a fixed gear artistic cycling bike more backwards than forwards, or mistakenly installing a unicycle's wheel backwards in the frame. Otherwise, for the pedal to unscrew you'd have to have a pedal bearing so damaged that the bearing drag dominates over the precession.
An alternate possibility is that the pedal's internal bearings could be loose, such that the pedal's axle is fixed to the crank but the pedal itself is wobbling on the axle. That may be correctable by tightening if the bearing balls are not yet lost or can be replaced, or you may need a new pedal.
Assuming it's a square-taper bottom bracket (which is standard on the Catalyst 3) you probably have to replace the crank as the attachment surface is now distorted.
See Fixed gear cranks loosening and Square taper crankset develops play after changing bottom bracket for examples of what happens when you ride on loose square-taper cranks - you usually have to replace the crankset since it will never tighten properly again.
Given the standard crankset for your bike is a Prowheel 24-34-42, assuming you are in the continental US you can get the replacement yourself from a smiley large online retailer named after a big river in South America for all of about $25 with free shipping.