Hello, and welcome to the forums!
There are many brands of nutrients out there, but probably the one most used by meadmakers are the products from Lalvin. They've got the largest body of literature regarding how to use them best, as well as their nutrient properties.
The major nutrient yeast need is nitrogen, which can come from both organic (in the chemical sense, derived from proteins) and inorganic (chemical salts). Diammonium phosphate--DAP--is the inorganic nutrient used most commonly in meadmaking. DAP is a pure nitrogen/phosphate salt. Organic nutrients are those like Fermaid-O from Lalvin; all the nitrogen comes from amino acids. There are hybrid nutrients like Fermaid-K that contains both organic nitrogen and DAP. For a more complete breakdown I'd recommend reading Ken Schramm's "The Compleat Meadmaker" or
THIS more advanced writeup here. Nutrients are a hotly debated and constantly evolving, so I'm sure many other mazers will offer their opinions.
How much to use? That varies considerably between meadmakers. A very easy to use protocol using 100% Fermaid-O is the
TOSNA protocol. Using all orgnaic nitrogen has certain benefits, but it does require ordering specialty nutrients online. If you can't get Fermaid-O, the same guy who made TOSNA made a
similar nutrient protocol using DAP and Fermaid-K. Notice that using these protocols effectively requires weighing the ingredients;
get a scale. They also absolutely require a
hydrometer, which you should be using in your meadmaking anyway.
