So basically, you hold down strings where the black circles are. If you play all the strings on the guitar (strum), you will get the exact melody sound.
If you play one string (pick it), you will get a note that is in the chord and will sound good with the melody. If you play single strings in a sequence (finger picking), it will sound very nice.
So basically, you hold down strings where the black circles are. On the violin, you can also play multiple string at once, but you likely cannot play all 4 unless your bow is bent. If you could, you would hear the melody. But since you can't, you will almost always be picking one string at a time. You will get a note that is in the chord and will sound good with the melody.
Note: You can play full chords when you have double X. ex) Fsus 3 3 x x (bow two strings at once)
It is harder on violin as you do not play the melody. Try singing over the violin while you are playing and you will notice it sounds correct although it is not the melody note. Or try playing to a CD (just make sure the CD is in the same key as your chords!). To check if it is the correct chord, play your violin like a guitar, strum all 4 strings and see if it sounds like the melody.
Note: You only need to hold down the strings you will be picking/bowing. I don't know the violin terminology. although it's good habit to hold them all so you can bow freely (by ear)
I actually have almost no musical background, but I am in computer science (which explains why I learned guitar from the internet). With my girlfreind's help, we mapped the guitar strings to the violin strings. Then I wrote a program to parse and reorganize my list so violinists could read it. We have played together and it does seem to work. If you find it very hard, you can try placing tape/string at the semitones lines so you know where to place your fingers.
The next step is to memorize the major chords (G,Em,D,C etc). And get fast at placing your fingers. Just like learning guitar :)
And if you are a violinist, try going to guitar sites and read up on picking. You can probably find a very easy way to adapt it to the violin.