I now have a zillion lenses. I didn't used to. I started with the K20D (my first dSLR, after many many years of shooting other stuff) with just three lenses: DA10-17 fisheye zoom, the lens that drove me to Pentax; DA18-250 superzoom, my basic lens; and FA50/1.4, my
gotta-get-the-shot lens. I arrived at these because, when upgrading from an advanced P&S, I asked myself: WHAT DO I WANT TO DO THAT I CAN'T DO WITH WHAT I HAVE NOW? The answers were ultrawide, ultralong, and ultrafast.
I had an advantage over total n00bs because I've shot various systems and formats over the decades, so I had some idea what to look for. But I think the same approach can work for anyone. Get zoom(s) that cover a wide range, and a fast lens for critical situations. My most minimal kit now is the above plus the Tamron 10-24 and a Raynox DCR-250 close-up optic. The FA50/1.4 is my only AF prime; the other ~200 primes are manual focus, and some good cheap ones. And only a dozen AF zooms.
I saw what I was shooting, at what focal lengths, and asked myself that WHAT DO I WANNA DO? question, and found some fast MF (manual focus) primes at critical focal lengths: 24-28-35-58-85mm f/2. Then I started buying enlarger lense to put on extension, and old slow lenses with unique rendoring, and many other cheap cheap lenses. Each has its own distinct flavor.
What should you buy next? That depends on your proclivities and budget. I strongly recommend the DA18-250 (or its Tamron twin) plus FA50/1.5 pairing as a start. Then the Tamron 10-24 (which I favor over the Sigma 10-20s and DA12-24). Going from there... it depends. Maybe a fisheye. Maybe a 90-105mm macro. Maybe a long mirror. To just shoot family activities, the cheap old F35-70 is hard to beat. If you find yourself shooting the DA18-55 at the wide end a lot an want something sharper, the Tamron 17-50/2.8 or DA21Ltd are great. There are many options. All you need is money.