To record vocals or acoustic instruments, you need to connect a microphone to your computer. There are different ways to do this. Many computers have a microphone connection built-in which includes a so-called preamp. The preamp is needed to amplify the microphone's signal. For better recording quality, you can use an additional sound card or external audio interface, most of which have a special microphone input.
If you already have an audio interface but it doesn't have an extra microphone input, then you'll need either an additional microphone preamp or a mixer with a microphone input to hook up to your audio interface. Your music instrument store can help you find the right device.
With electric guitar and electric bass, you can record directly into Sequel without needing a microphone. Many audio interfaces on the market today have a special instrument input (also called "high ohm" or "hi-z" input) that are specially built for recording electric guitars or basses directly. If you have an audio interface without a special instrument input, you can add a separate guitar preamp to your system. For help in finding the right interface, ask at your music instrument store.
Electronic instruments like keyboards and synthesizers can be hooked up directly to the stereo input in your computer or audio interface. A special hardware or input is not needed to record them in Sequel.