This is a great camera with a lot of advantages for a beginner. It's dead simple and very reliable. It doesn't even need a battery, except to meter. And the meter is great — a simple, direct needle which indicates the overall exposure of the whole frame. I find this much more enjoyable to use than the LED indicators used on newer fancier cameras. Combined with the simple, direct manual control, it's very easy to learn what you're doing quickly.

The viewfinder on the K1000 wasn't pro-level for its day, but it's a prism rather than a mirror and because film is larger than APS-C sensors, it easily outclasses what you'll find in entry-level DSLRs, so that's nice too.

Pentax hasn't significantly changed the lens mount in decades, so you have a lot of lens compatibility. The main caveat is that newer lenses (with the "DA" designation) won't have an aperture ring which leaves them somewhat crippled. Conversely, when you use an old lens on a new DSLR, you have to press a button to take a meter reading. That limitation aside, it's nice that lenses you might get for the K1000 will also be useful on newer cameras.

Pentax's classic manual lenses are very well regarded, by the way. The 50mm f/1.4 and f/1.7 are some of the best. The K1000 often came with the 50mm f/2.0 (it'll say 1:2 on the lens; on these lenses, that's the aperture designation, not a macro magnification label), which is really sub-par compared to the others. You should be able to find one of the others relatively cheaply (under $50 for the f/1.7), and I definitely recommend that upgrade.

I recommend the 50mm in particular because it's the "normal" lens for a 35mm film camera — a very versatile and useful focal length that you can use for just about any type of photography. You may want to get a longer (higher focal length) lens for a more "zoomed in" view, particularly for portraits. There are a number of older Pentax 85mm lenses which would fit this well. There's also an easily-available 135mm, which often came in a kit along with the camera and the 50mm f/2; like that 50mm it's not top-notch, but it does have a certain charm and is good for head-only portraits. You may also want a wider-angle lens for landscapes or for getting more context.

You don't need any of these — I mostly just treat my K1000 has having the 50mm f/1.7 permanently attached — but if you really want the most flexibility, you want a few choices. You don't need to worry about covering every focal length or collecting a set (unless you're into that sort of thing), but get what matches your use. (There's plenty of older zoom lenses available, but generally their quality isn't up to what we're used to with modern zooms — sticking to prime lenses is a good bet.)

There's the larger issue of whether film itself is still relevant or good for learning, but we have other questions and answers addressing that. (See (How) should I start with film photography?)

Taken as a given that you do want to learn to shoot film, the K1000 is a great choice. It's still a staple in college photography courses, and because of that and because so many were made, if you didn't have one already it'd be easy to find one used.

Since a photograph is worth 1000 words, here's one of my favorite family snapshots taken with the K1000 and 50mm f/1.7 lens, on Kodak Portra NC 400 (pre-2010):