for a Pentax K1000
You have a 50mm f/2 lens, which is called a prime lens or the "normal" lens. This lens sees things pretty much the same way that you do. Most people are interested in getting a zoom lens for dramatic close-ups and some people are then interested in getting a wide angle lens. Some are also interestedin getting a macro lens. Since you are new to this whole thing, I'll make a couple of suggestions in the lower priced range. You can probably find similar lenses make by Sigma, Tamron or Tokina that will cost even less and they will be decent lenses. You will also find some Vivitar and Soligor lenses from that era that are okay to use. Even on the low-priced end, I prefer to but lenses made by the maker of the camera so that all of your lenses will work the same way. You won't have to focus to the left on one and focus to the right on another, etc. Genuine Pentax lenses may be labeled Pentax or Asahi Optical. Usually, they will also say "SMC" in the name.
Now, these lenses may not test up to the same quality as the really high-line Pentax lenses, but they are a starting point.
A nice zoom range is provided by the 75-300mm zoom. 300mm is a pretty good zoom, meaning there is a lot of magnification. It's like using a small telescope or some 6-power binoculars. It costs about $130.
http://www.bhphotovideo.com/bnh/controll...
If you want to have a multi-purpose lens that goes from wide angle to normal to telephoto, you can get the 28-105mm zoom. 85-to-105 is a nice focal range for portraits, so this lens covers that as well as giving you a decent wide angle all the way down to 28mm. This might actually be the best additional lens to buy, as you will get an idea about the differences between wide angle and telephoto. You can add a 1x or 2x "teleconverter" to get the range out to 315mm, too. The lens costs about $190.
http://www.bhphotovideo.com/bnh/controll...
Now - these are auto-focus lenses that I don't think the K1000 supports. You should still be able to use them in manual focus, but you could save more money by finding some all-manual lenses on eBay. You may have to ask someone's help in choosing them, though, because there are so many available.