There are tons of software programs, some free, available for download on the net that can help you create a photo collage that works. But in actuality, a little work with your friendly Paint program, usually installed on your hard drive, is all you need. It's not as easy to control the final image size, but putting together a collage in Paint is a piece of cake.
Open a new Paint file, click the bottom right-hand corner dot and drag down and to the right to make the page the size you want. You can repeat this process to make it bigger and bigger. If you want a background that's a different color from the automatic white one, choose the color you want from the color toolbar, click the paint bucket icon, then click inside the page. This color can be changed later using the same process.
Get all the photos you want together in a single folder, using thumbnails to view them at a glance. Right-click on the first photo and choose "Open with Paint." From "Edit," choose "Select All," then "Copy." Go back to your original collage page and choose "Paste" from "Edit." You're photo will appear on the page and you can drag it to where you'd like it.
Continue the process with the other photos until you've finished your collage. By working (read, "playing") with your picture programs, you'll quickly become adept at your new hobby...and entranced by it as well!