One of the most popular sites for guitar tabs is ultimate-guitar. I use the site myself and have contributed tabs as well. This site also hosts files with just the chords to a song laid out over the lyrics. This makes it great for learning songs on other instruments too, like the ukulele or piano. There is a button on the upper-left of the screen when you open a file on the site that looks like this:
If the song has chords that are too difficult, what you can do is click the down arrow until the chords are more to your liking, then place a capo on the guitar at the fret number of however many times you clicked the down arrow. For example, suppose I want to play "Hotel California" by The Eagles and I look it up on the site only to find that the chords are way too hard for me. I try the transpose button and find the transposing down twice yields much easier chords to play (A minor instead of B minor, E instead of F sharp, etc.) so I use those chords instead and put a capo on the second fret. Now I'm playing the song in its original key with easier chords.
Suppose instead that you want to play and sing a song that you really like but the vocal part is WAY out of your range. No worries, we can transpose a song to make it easier to sing too. This time lets say you want to play "When I Was Your Man" by Bruno Mars. He sings really high. That's okay though because we can transpose the song to make it easier for us to sing. Click that down button 5 times (this is what I use when I play it, easy chords too) and then try it. What a difference! It may take some practice to mentally transpose the vocal part but it isn't that hard. Just play the chords through a few times and you should hear it.
This also works for a vocal part that's too low. It's especially useful for a female voice attempting a song sung by a man with a low voice. Just hit that up arrow until you can sing the song comfortably. Generally, you'll need to transpose at least +2 to make a noticeable difference. This also works if a song already calls for a capo by just taking the capo off and playing the same chords. For example: "Collide" by Howie Day is played with a capo on the 4th fret(3rd for his acoustic version). If you take the capo off and play the same chords, you can sing the vocal part higher to match the guitar part.
You are now armed with the knowledge the pros use to transpose songs! Check out this video where I transpose a bunch of songs to the same key (G) and play them all with the same chords: