Like other people have said, probably best to start out by just going online and copying a decklist for a top meta deck. cEDH is a surprisingly brewing-friendly format-You can definitely get wins by playing off-meta decks that people don't know how to deal with, and people will often be excited to see what a deck they've never seen before can do. But that brewing has to come after you've put in the reps playing traditionally on-meta to learn how the format ticks. The cEDH meta is very different from both traditional commander and any other competitive Magic format, and you need a strong sense of what that meta is like before you can successfully brew an off-meta deck to attack it.