Belts, Pouches, Boxes, Sheathes, etc.
Basic belts (which are fairly easy, although doing a nice, quality piece requires several finishing steps that most people leave off, like beveling, edging, forming the belt around the buckle, etc), pouches, knife sheathes, and so forth. To get started in this, (We suggest you pick up a copy of Al Stolhman's booklet on Handsewing Leather. A Tandy Leather hobby shop should have a copy of it, or be able to order it for you.)

Boots & Shoes
If you are interested in shoe and bootmaking, specifically for medeival recreation (in the SCA) and we've been collecting information (some good, some not so good) about it, but we have yet to get much practical experience with it. You can generally find some basic introductory material and kits on moccasins and the like at Tandy leather, most of these kits only get close, because like commercial, mass-produce shoes, the leather is cut to standardized sizes, and your feet generally don't come in those standardized sizes.

Garments
We don't do a lot of garment making with leather, because it's much more like sewing cloth than like the rest of leather working, and we've never found sewing cloth too easy. The leathers you use are generally a heck of a lot more supple and lighter weight than what you use for other activities, and aside from the peculiarities of the material, it's just like making clothing. Hence, if you're interested in this, you'd be better off learning first about sewing clothing, then learn about how to bring leather into your repertoire.

Everything Else
Of course,  we've only touched on some of the things leather can be used for. There's an immense field of saddle-making and harness-making, not to mention whip-braiding, and that's just the first three things that popped into my head. There are an astounding variety of possibilities - and indeed of examples throughout history - for making things with leather.

Tools
For construction, we suggest you start with at least the following:
Knife - a sharp knife with a small blade. A utility knife or "box knife" (called so becaue they're usually used in shipping and such to open cardboard boxes). Anything that fits that general profile should do: mostly handle and a half-inch to an inch of blade, so the cutting tip is near your grip and easily controlled for precise cuts, yet allows you apply strength when necessary for heavier leathers. Some people use Exacto knives.