You will need to google if you want to use mods on Rebirth or Afterbirth.
Otherwise you need atleast Afterbirth+ to use workshop mods.

Then forget about the workshop entirely and go to moddingofisaac.com instead. Don't expect too much from Rebirth and Afterbirth mods though, modding is quite limited on these versions.

honestly, the first time I played this game, I just bought all the DLC'S on sale with it, and I never have to worry about this.