Do I install a 15 or a 20 Amp GFCI?
The standard 15A GFCI receptale are rated for 20 amp feed thru so you will have no issue with that. But make sure you watch out the line and load connection and how many receptales downstream will be on this circuit. If this circuit do not have any type of GFCI at all before you start that make sure you mapped out the circuit to see where it will affect any other circuits like other light or receptales. Once you mapped out then you can able pretty much pinpoint where is the starting point to install the GFCI receptale If more than two pairs of cables in there post it back then we can see where go from there. If you have test light that will really help you to determed which is line and load by testing the hot and netural wires with test light once you know then the rest should be pretty much on target.