You need an audio interface with more than two input channels/preamps. Or, you can use a mixer that accepts say 8 inputs, and record the stereo mixdown through your Lexicon interface. That's not ideal, better to get an interface that supports enough channels so you can record them independently and then mix them to taste after the fact via your recording/mixing software. Generally speaking, you don't need a mixer at all - if your interface supports enough inputs, your DAW software is the mixer (a virtual mixer).

Hello Drama

Welcome to our Recording Forum!

Please, search the forum for all the info you gonna need. It has all been described, many times, what to do and what to buy.
A visit in a music shop and a few manufacturers web pages for some guidance would be of great help, too.
It is a difficult task to teach recording from bottom level. Do some reading and gather more budget. You'll gonna need it if you want to record in a decent way,
other then placing a 2-track handheld on a chair infront of your set. What is your budget, now? There are covertors from 200 to 12.000 Euro. What microphones do you have?