The best thing you could do would be to get yourself started by going out on a few trips with someone who knows what they're doing. Doesn't have to be an outfitter, but that would be nice. Might get you spoiled, though, with them doing all the packing, cooking, and wrangling.
A good friend, who doesn't mind teaching you is the best way, in my opinion...and cheaper.
If you don't have that available, I'd start out just doing a few overnighters without a pack horse. Pack all you need for one night on your horse. It will teach you what things are essential and what you can leave at the trailer. Once you get comfortable with that, then think about packing one of your saddle horses with a saddle panier, like Painted Horse suggested, for a trip of two or three days. Saddle paniers aren't all that expensive and they're great for short trips.
Then, if you find you really do
enjoy packing, you can step up to a pack saddle and a dedicated packhorse or mule, and start putting an outfit together. Get good gear and it will last about as long as you will.
There are several good books out that go over all aspects of horse packing. I like "Packer's Field Manual" by Bob Hoverson, but then I have some experience. There are probably others that are better for your needs.