If you have a tape deck with a stereo line out (two RCA Jacks) and a standard consumer computer with sound hardware, and some software to control the operation, it's a piece of cake. Most standard consumer machines have a sound card or a sound chip built into the motherboard, and there will be a 1/8" stereo minijack on the back labeled "Line In." (Sometimes this is not labeled that clearly, they'll use a stupid little icon instead that's maybe not that obvious.) And most machines offer at least some rudimentary recording application that lets you take the signal coming in the line In and save it to a file. There are also freeware, shareware, and payware applications out the wazoo to choose from.
To connect the deck to the computer, you need a cable that has two RCA plugs on one end wired to a single stereo 1/8" miniplug. Plug the RCAs into the left and right Line Outs of the tape deck, and the 1/8" miniplug into the computer's Line In. You will probably have to fiddle with the input levels to get them just right.
One more note: you do not want to use the minijack labeled Mic In. This is a mono input that has a preamp circuit to boost the tiny signal from the junky plastic mics.