People talk in hushed tones of the so-called "ginger beer plant" which used to get passed down from generation to generation, and around the neighbourhood, and was basically a continually regenerating starter that could be used to make natural soda-type ginger beer.
Although you can buy different starters to make ginger beer, using any kind of yeast is cheating - the original "ginger-beer plant" seems to have been some rather specific yeast/lactobacillus combo and when you buy a "ginger beer plant" I really don't know what you are getting.
But I followed some instructions around the net and got my own ginger beer starter going, and this is my experience (I kind of assume you know about making sodas and things, so the instructions are a little compressed)
All you really have to do is put some raw, unpeeled ginger (the peel MAY contain some kind of useful yeasts) in a bit of unchlorinated water in a clean jam jar and add some sugar, and keep adding a little water and a little sugar every day, leaving the lid off all the time, with just some gauze or something over it, preferably away from sunlight. After a few days you should get some fizzing - and if it still smells good (gingery) you are in business! If not, chuck it and start again, it took me my second attempt to get it going, the first time it either went stinky or didn't "take", or both, I forget which now.
This is what mine looks like at the moment.
Now you can build it up for a few more days and then use a cup of it as a starter in about a gallon of room-temp dechlorinated water in which you have previously boiled about a half-pound (a matter of taste really) of grated, boiled ginger and however much sugar you prefer - 3 to 6 cups seems about right, I go for the lower end of that. I also add lemon juice before bottling, to taste, probably the juice of two lemons. Bottle in empty plastic Coke bottles or similar and leave at room temp for 24-48 hours (I use plastic, it never lasts long enough to be worth bothering with glass), and wait till they are as hard as an unopened coke bottle. Actually I like it a bit harder than that, i.e. very gassy, but don't go crazy or you'll get either an exploding bottle or one that is impossible to open without causing a geyser. After that it MUST be kept in the fridge or it will kick off again!
It really carbs up a treat - incredibly refreshing. That's the thing though, I have no idea what strain of yeast is in there, your results may differ, it would be interesting to find out, and no idea whether that is the same "ginger beer plant" of tradition. All I know is it's spontaneously fermented and tastes great!
I have just resuscitated my jar of the starter (pictured above) which was in the fridge for 3 or 4 months over winter. I added a little more sugar and water and left at room temp. It smelt a bit off for a few days, like the ginger pulp on the surface had gone a bit rotten, but then the smell cleared up and it started fizzing again, and produced another perfect batch of ginger beer, ready for the warm temperatures that have already begun in my part of the world