You will likely need the kit - which may or may not include a can of freon, a can of stop-leak, a hose assembly with a fitting at one end for the can and at the other to connect to the low-side line, and if it's a better one, a gauge. Whatever you do - do not put the stop leak in. Will probably want to add freon to get the low-side pressure up in the 40-45psi range for your R134 system.
While low freon is the culprit in many cases, it's not the only cause. One may get lucky, or one may cause more problems. To really understand the problem, need to put a full set of gauges on the system - both the high-pressure and the low-pressure sides. Rather than purchase the gauges, it's often less expensive to let a professional do the work.
Another point is that air conditioning systems operate with high pressures and extreme temperatures - both hot and cold. If you don't know what you're doing, there is a much greater risk of damaging equipment or causing physical harm that will cost more than having a pro do the work. If you choose to try this - please wear safety goggles and hand protection as a minimum.