Get a line level adapting cable from someplace like Radio shack. That will change the speaker level output of your tape recorder to line level for your computer.
Plug it into the speaker output of your recorder and line level input of your computer. Use just about any recording software ( a lot of free ones out there plus the ones that come bundled with your computer) then to convert it over to what ever you want. Almost all of the software will do mp3's and most other output types.
Start the recording software and then hit play on the tape player. You will need to experiment just a little to find the right playback volume on the player so the mp3 sounds decent once recorded and converted over. Just start about mid-range for the volume control on the tape player. That will generally get you in the ball park for decent playback.
You may find out you will want to break the lengths of of the recordings up instead of doing each one whole. You can end up with some huge files recording full 30-60 minute lectures in one piece.
That's the simplest method I know of. You can buy adapters(with software) that will plug into your cassette player earphone jack that use a usb plug to go into the computer also if you don't have a line input on your laptop.
If the operating system in use is windows, use windows sound recorder to record the sound. Simply plug the tape player into an audio input jack usually built into the CPU motherboard and hit the red dot record button on the software control panel and hit the play button on the player to record the sound. The best quality available on windows sound recorder is the PCM format with a sampling rate of 48KHz and a bit rate of 93kbps. The sound recording volume of this software on the other hand will depend on the input volume of the tape player. Estimation is needed when adjusting the volume to ensure that no noise will be added to the recorded audio. Additionally, windows sound recorder has a limited time of 60 seconds allotted to it, so to increase the recording time, make a blank record by hitting the record button even when no audio is playing and every time the recorder hits 60 seconds, hit the record button again to extend the recording. When the desired recording time is reached, save the file and simply open the blank record again to record the audio over it. When the audio is saved, it will be saved as a WAV format which will occupy 10 times more space than an MP3 file so use an audio file converter software such as ImToo MPEG Encoder Wizard 3 to convert it into an MP3 audio file.