Hi and welcome,
If your Audio Device has an Hi-Z (Instrument) input, you can plug your Guitar directly to this input and enable Hi-Z.
Then plug your mic to the other input. In Cubase add 2 Input busses (in VST Connections or Audio Connections in Cubase 10). Add 2 Audio Mono tracks with 2 different inputs, and you can start to record.
Please, watch a Quick Start video tutorials on the official Steinberg YouTube channel.