Interfacing

[rug]

So Samara is in the process of recording ourselves. We have mics and headphones and are running thru an 8 channel mixer which runs to the computer(Using Acid 5.0 software). My guitar amp has an output to run direct into the board but when recording, the guitar has to be turned off when recording the drums (on the board but it can still be heard in headphones by both who are playing) or it will record on the same track as the drums. How can these be seperated? I think an interface will do this but i'm not sure and if it is an interface that we need, what kind?? Ive been looking at some online but there are so many. Any input would be appreciated and please keep it as simple as possible...this our 1st time at doing this so this is all kind of new. thanks.