Watch the The Water is Wide online guitar lesson by Stephen Bennett from Harp Guitar Basics
All pretty straightforward.
One other thought here: really what I'm hoping you'll be trying to do at this point is make stuff up. Except we have a better word for that musically. I'm hoping you'll improvise a bit. Now I realize that I don't know anything about you individually. Maybe you've improvised a little bit with your guitar before, maybe you haven't. If you have, you already know that the way to get better at it is to do it more. Do not be afraid Grasshopper. Go out on a limb. Sometimes you get a better view. Sometimes you accidentally saw it off behind you. Doesn't matter. Do it again.
Here's an idea: Try playing the chords to the tune – and hum or sing the melody as you do it. (If that's hard work, keep doing it because you want to be able to do that). Then try singing the first phrase the way you hear it in your head – but then finish the phrase in some other way. In other words, you'll be doing the first part as you learned it and then making up the second part.
Project that throughout the tune, from phrase to phrase. It doesn't have to be anything dramatic. Something as simple as changing one or two notes can be enough to put your own mark on a tune. Maybe it's just a matter of sliding into a note when you weren't before. Maybe you hit a harmonic instead of a fretted note. If I tell you exactly what to do, you're not improvising, so you try something.
Listen to how 2 different singers or instrumentalists play the same tune. Different versions will generally be similar enough so you know it's the same tune, but different enough so that you know it's different people. That's all you're after here.
Pay attention to your mistakes. Sometimes they're actually nice. Make them again if they are. Sometimes they hurt. Pay attention to those to.