There are no common keys for the baritone voice or for any particular voice, for that matter. It depends on the range of the specific song in question and how it compares with the range of the voice. Lox's suggestion of finding the lowest note that you can comfortably sing is a fairly good starting point, but you should go on from there.

First things first:   
1. Find the lowest note you can sing comfortably and still get a good tone.
2. Find the highest note you can sing comfortably and still get a good tone.

That's you range.

Let's say, for sake of example, that you (baritone, you say?) can manage an octave and a fifth comfortably. Low note, around A, the open 5th string of the guitar (you can growl down to the low F or E, but you can't get any decent volume on them). High note, E, the open 1st string. You can actually squeal up to the G above that, but it either wants to break into falsetto or otherwise doesn't sound real good, okay? [Your actual mileage may vary, but this is just an example of how to do it.]

Say that, in a song book, a song that you want to learn is in the key of G (one sharp - #). The lowest note you find in the written music is a D (open 4th string) and the highest note is an F# (1st string, 2nd fret). That key is high for your range. So you want to count down scale steps from D to A (D, C#, B, A), the lowest note you want to sing. Then count down an equal number of scale steps from G to find out what key that would put you in:    G, F#, E, D. Bingo! Key of D. If you do the same countdown on the top note (F#, E, D, C#), that puts the top note of C# that you'd have to sing well below your voice's comfortable top note of E.

Okay, now try it in the key of D and see how it goes. Since it's fairly well within your comfortable range, you have some latitude. Try it up a key, in E and see how it goes. That puts the bottom note at a B and the top note at a D. That might actually sound better. Experiment.

That's it.

If you don't read music, you can do more or less the same thing by checking your range against the guitar (assuming it's in tune). Figure your voice's top note and bottom note in relation to strings and frets. Then, pick out the melody of the song on the guitar. Once you know how high and how low it goes, you can move it up or down until it fits within your comfortable range.

I hope this helps.

Don Firth