I own an ancient copy of Photoshop Elements, but I am so embarrassingly incompetent with it that I use it only very occasionally. And I'm not really in the mood to spend a bunch of money on a new version that I ALSO will not know how to use.
So, my tool of choice is Picasa, which is Google's free photo editing software. It's not particularly fancy, but neither am I, so we're pretty much a match made in heaven.
If you'd like to make a photo collage in Picasa, it's pretty straight-forward.
Open up Picasa and in the search bar, type in the name of the folder containing the pictures you want to use (mine import automatically when I create a new folder in My Pictures or My Documents, but if it doesn't, you can can manually add the folder by choosing "Add Folder to Picasa" in the drop-down File menu in the top left-hand corner).
Then select the photos from that folder you want to use in a single collage (hold down the Ctrl button and use the mouse to click the photos).
Click on the "Create" drop-down menu on the top toolbar and choose "Picture Collage."
Now you have all your images in the collage maker and you're ready to goof around with them to your heart's content.
You can choose different arrangements (Mosaic and Picture Pile are my favorites), and once you've selected those, you can modify THAT. For instance, in picture pile, you can choose which images are on top, how big each one is and where on the screen they sit. In Mosaic, you have a little less control, but you can choose which photos go in which boxes.
If I'm doing Mosaic, I like to do a little grid spacing, so you have some white space between each image, which I think makes it easier on the eyes.
And then you can resize the whole thing, either with the standard settings (square, 4x6, etc) or with your own ratios. I do find you have to play around with the ratios quite a bit to get the right one for your particular pictures.
You can also edit an individual picture (if you want to change the contrast or coloring a bit to match other things better or recrop it, for instance), by right-clicking on the specific image and choosing "View and Edit."
If you aren't happy with it, you can always push the "reset" button and it'll put everything back in the original spots.
Once you wildly thrilled with the whole thing, push "Create Collage."
Now you have the options to edit it as you would any normal photo including adding text to the top of the whole thing.
When you're done, push "Export," select the folder you want it saved to and, voila, collage.
Let me know if you have questions or need some trouble-shooting help!