Investing in an audio interface would be the way to start, especially if you don't have a mic input. You can get a really basic one, don't really need anything high end, you get a lot of quality stuff at reasonable price points these days.
That said, getting a better interface will improve things like latency and the quality of the conversion of the audio signal into digital. Alternatively, you could hook up with midi cables and play into the appropriate software like GarageBand or something like Superior or BFD to get the best sounds available, but this would be heavy on computer resources and needs better hardware.
All said and done, do read up on how to record before you make any decision. Planning purchases and setups will be of benefit in eliminating the need for a replacement further down the road.
- Reggae Mangle