I have just started reloading and was wondering how I go about cleaning my brass? I have a tumbler with both walnut and corn cob media. Do I need to add some sort of "cleaner" to this media or just toss the brass in and and start tumbling?

just throw them in. they doo make a recharge liquid but i use it sparingly. i like the walnut better than corn it tends to last longer

Ditto, throw it in and let it tumble. And Ditto, the walnut lasts longer and gives good shine. The corn cob stuff will give a better shine, but doesn't last as long, and takes a lot longer.

I toss in a couple teaspoons of Comet every now and then, it'll polish'em up real nice.

Jay

I us Flitz and corncobs.It make nice shinny brass.

This may be a dumb question, but is it necessary to clean the brass before reloading? I haven't ever reloaded before, just want to know.

It's not actually necessary. I only say that because I handloaded and reloaded for years and never owned a tumbler. I would de-prime, neck size, clean the primer pocket, and load back up. The only brass cleaning I did was to wipe the re-sizing lube off with a towel after the round was loaded. I never had any issues at all. That's not to say cleaning is bad or a waste of time because it is neither. It's just not really required if you start out with clean brass. JMO.

Eventually the cases will get so dirty that they will leave grit in the die. This will scratch and score the inside of the die. I loaded for a while before getting a tumblers myself, I think everyone does. But it is better to clean the cases if possible.

Jay

I like to tumble my brass when the hulls take on a dirty dull look to them...after about 4+ loadings depending on calibers.
Before I bought a tumbler I used to use some liquid stuff that I could soak deprimed unloaded brass in overnight...dunno if it is still available...can be a PITA to use and flush the solution off the brass...and frequently had to dry the cleaned brass the oven on a cookie sheet - wife never did like or understand that...drying brass in the oven also left a residue on one side if the brass was very wet initially. A buddy used an airhose off his home compressor to blow all the cleaning fluid and rinse water off and that worked OK too but it needs to be done outdoors.
Ron