DEBBIE, the system already had a leak=oily hoses, so it's already DISCHARGED.
besides the R134a freon that you need, you'll probably have to add some special OIL to it as well or else you risk damaging the compressor!
using one of those freon kits w/ some sealer and uv dye already added may or may not help, depending on how bad the leak is. for those DIY kis, the valve is only going to be attached to the LOW side of the system (one size - one fit) all the r134a systems will have 2 sizes so you can't tap into the high side with those DIY kits
but then, you'll just be guessing at the amount of AC compressor oil to add as well... so a pro may be needed to complete this job properly
when the leak is found & repaired w/ a new gasket/ o-ring/ or whatever part that had failed, the professional will evacuate the system & keep the vacuum pressure for a few minutes to confirm no more leaks after the repairs.... this also makes sure there is no residual moisture to rust out the parts from the inside. then he measures the correct amount of freon and "recharge" the system.