What you are going to need is a program called Blitz3D, which will let you edit and compile the source ".bb" files that contains the code for the game. You can find the source files for SCP v1.3.8 on github
here.
I don't remember where I downloaded Blitz3D from, but if you google around or ask someone, you might find one. You will need to put the files that is in maveless_bin.zip (located in the SCP source folder) into your "/Blitz3D/Bin" folder.
Aside from that and basic coding knowledge, you should be good to go. To edit item names, edit the file "items.bb" and to make new items, edit "main.bb" and "items.bb" together. If you need coding help, either compare the code already compiled in the game or ask around. That should be all you need to get started.