Welcome to Leatherworker.net!
First off, NOBODY (except maybe Peter Main) decided one day "I think I'll turn out a masterpiece in leather." We ALL started modestly with pieces that we would now probably consider hideous (though to be fair, a lot of new members are turning out better work because of knowledge base in this site). I feel pretty confident in telling you that you'll probably spend more in tools, materials and especially TIME, than you would on ordering the nicest custom holster you can find. But, you won't have the satisfaction of knowing YOU made it. It's a pretty rewarding addiction hobby, and you'll find yourself exploring different avenues than you originally thought.
Ferg has it right- do a lot of reading on this site, there's a ton of instructional material and even more inspiration.
The basic tools you'll need to make a holster are as follows:
Leather, a needle, thread, something to cut leather, something to mark leather, something to punch holes in leather (so you can get the needle through it)
Things that will help you make a better holster: the right kind of leather, in this case- vegetable tanned, stitch hole marker (usually a stitch wheel), stitch groover, a good diamond awl (blade shape), possibly wing dividers, a second needle so you can saddle stitch, an edger, a boning tool, edge slicker, the right tools to cut the leather, glue, snaps, dye, oil or conditioner, a finish for the completed holster, and a pattern. Add more leather for the 'new' holster you make to clean up all the little mistakes you make on the first one, and more time.
If you want to do any tooling, you'll need stamping/carving tools including a swivel knife...... LOTS OF NEWSPAPER to keep the drips of dye off of the table/counter/carpet.
Just think of all the fun you're about to have!
The good news is that you can get a lot of the basic tools in the little kits available from one of the vendors, and you can call the leather suppliers listed at the top of the page and tell them what you want to do and they'll help you get the right leather....all online.
We'll be here to help out when you need us,
Mike