The name "USB audio interface" kind of tells you what you need to plug it into.
As for the guitar and vocal thing, well, there are several things you need to understand.
There's this signal level called line level, which is the level used to interconnect gear (mixers, recorders, effects devices) htrough cables. In order to be effectively routed to a recording device, everything needs to be made to be line level.
Electric guitar pickups have very small signals (called instrument level) that are amplified to line level by the preamp stage of a guitar amp (the power amp stage them amplifies it so it can drive speakers). You can't plug a guitar straight into a line-level input, it's not a line-level signal.
Microphones also have very small signals that require preamplification to get to line level. (In live use, a mixer would send its output to a power amp to turn all line level signals into a powerful-enough signal to drive the PA speakers.) Like a guitar, you can't plug a mic into a line-level input, it's not a line-level signal.
Audio interfaces come in different types. The simplest accept some number of line level signals and digitize them so they can be recorded and manipulated in the computer. Others include a preamp that can accept mic level and/or instrument level signals also.
As far as I know there are at least a couple of Edirol USB interfaces. The cheapest simply accepts a stereo line-level signal, like the output of a cassette deck or CD player.
If you already have the front-end stuff needed to get microphone and instrument signals to line level -- namely a mixer or mic preamp and a direct box or direct line-level output from a guitar amp or (the tied and true and best way) a mic that you just stick in front of the guitar amp speaker, then an interface like this will probably suffice.
If not, you might wish to consider one of the more expensive models that boast a mic preamp. I think they even have one that has a guitar input with their COSM amp modeling technology built into it.
