If you already have a cassette deck, all you need is a 2-RCA to 3.5mm stereo mini cable and and an audio editor. Audacity is a good free audio editor, although I use SoundForge.
Some of these packages give you very little control and just record automatically. Be leery of automatic track separation and automatic hiss removal. The tracks will be 'separated' based on detecting silence - but what level of silence? -50 dB, -60 dB, etc? Each tape will be different. Simply removing 'hiss' by applying a 'noise gate' (a function of most audio editors) will not actually do much for hiss and sometimes will end up removing actual sounds that are low in level but very audible.
If you want really good results there is no way around the learning curve of learning to use a good editor and doing it yourself manually.