The surprising health benefits of kefir, a unique fermented milk drink laden with probiotics and yeast species, mean it will help you out whether you're dealing with gastrointestinal conditions, hoping to reduce your cholesterol levels, eager to shed some pounds, or simply keen to strengthen your immune system. 
You don't need to actively be looking for better health to make kefir part of your diet, though, as it also has plenty of nutritional benefits. Being made with milk, kefir will give you B vitamins, calcium and other minerals, amino acids, and protein in kefir.  Kefir additionally gives you a strong dose of probiotics, including one — Lactobacillus kefiri — that is unique to kefir.  Finally, kefir simply tastes really good. The more you drink, the more you'll grow to appreciate its flavor.
Using A Kefir Starter To Make Your Own Kefir
Kefir "grains" are a complex symbiotic mix of yeasts and friendly bacteria, with some random added extras like sugars, proteins, and fats . They live in a glass or plastic jar, you add milk to them, and they work their magic to turn your milk into kefir. After a fermentation cycle of roughly 24 hours, you strain the kefir grains through a plastic strainer (or a cheesecloth if you want to), and start the process anew by adding fresh milk.
Caring for your kefir grains has a bit of a learning curve at first, one that I hope becomes a little easier by clicking on that link just there, but it is ultimately really simple. Basically — feed your kefir grains new milk every day and keep them at room temperature, and they are guaranteed to produce kefir. Day after day, for many years if you keep it up. What's more, your kefir grains will quickly grow to the point where you can share them with your friends, relatives, and anyone else who wants them, as well.
It's best to pick your new kefir grains up from someone local, but some people will ship fresh kefir grains with a bit of milk around them in the mail, in a Ziploc bag or similar. Kefir grains are also available in dehydrated form, in which case you will need to rehydrate them with milk before they are ready for use.
In case you were wondering, nope, you can't create or grow kefir grains from scratch. You can, however, try to make kefir without kefir grains.
What About Powdered Kefir Starters?
Powdered kefir starters are lab-created using spray drying or freeze drying — a process during which some of the many microorganisms contained in kefir grains will inevitably be lost . Powdered kefir starters are commercially available in some countries, including the United States, and they create kefir fairly consistently when cultured at room temperature, just like kefir grains, when you follow the provided instructions. While such kefir starters can be re-cultured a few times using a portion of the initial finished product they are, unfortunately, ultimately a short-term gig. This means you will have to repurchase powdered kefir starters frequently if you wish to keep drinking kefir.
Can You Make Kefir From Old Kefir?
If you can't get your hands on kefir grains or powdered kefir starters but commercially-produced kefir is available in a store near you, you could still try to make kefir at home. You do this, "random people on the internet" say, by adding roughly a tablespoon of store-bought kefir for each cup of milk you're hoping to turn into kefir, and then letting it ferment at room temperature for 24 hours. Folks who've done this say that results are inconsistent, but you may just end up with nice kefir.
Not wanting to leave you with hearsay, I decided to give this a go myself by attempting to ferment a cup of milk with about four tablespons of kefir. (More is more, no?) I live in a Mediterranean climate and it's currently summer — so hothothot! My mixture turned into a mess of whey and casein really quickly. Kefir with kefir grains, mind you, can also separate, but mixing it with a spoon reconstitutes the parts. That wasn't the case for my little experiment. I ended up, I think, with nothing other than gone-off milk. (I wasn't brave enough to actually taste it.)
I asked a friend who also makes kefir with kefir grains but who lives in a colder northern climate to repeat the experiment, and her mixture seemed to do nothing at all. She did taste hers, and after 24 hours, it appeared to be just milk, minus the spoiled part.
A sample size of two hardly constitutes scientific research, but I think we can conclude that this kefir-making method is at best hit-and-miss.
The Bottom Line
Kefir grains are the gold standard for people who want to enjoy home-made kefir on a daily basis, and I'd suggest you try to get your hands on some. Powdered kefir starters are a nice alternative, and maybe a first choice for those people who only want kefir occasionally. If that's not available either, trying to make kefir from old kefir is an interesting experiment that may or may not yield the results you're looking for.