What's the best software for making a collage?

You do not state whether you want an app for your phone or a program for a desktop/laptop computer.  I'll assume you want a program for your computer and not an app for your phone.  Doing it on a computer is much better anyway.

What makes a image-editing program great for a collage is being able to work in layers.  Layers just means that various images (or pieces of an image) are stacked one on top of each other.  Working in layers allows you to edit and move each photo separately from all other photos within the collage.  Photoshop will do this, but what if you do not want to pay the $10 per month subscription to use Photoshop?  Well, there are other programs that do a fantastic job such Luminar.  There are free programs such Gimp (free at gimp.org) that also one to work in layers.

Simply create a new file with the size and resolution that you want.  Say you want a final image that is 8" x 10".  Create a new file with the dimensions of 8" x 10", this creates a file that has a canvas size of 8" x 10".  The "canvas" size is simply the height & width of the file.  The canvas size is separate from the resolution.  Set the resolution to that of the highest resolution of the images that you'll import or copy into the new file.  As you copy & past new images or parts of images, a new layer is automatically created.  Clicking on a layer in the layers tab will allow you to edit that photo or move it without affected any of the other images.

When you're finished with the collage, save the image as TIF file which is a file type that will retain the various layers so that you can come back to the image at any time and rearrange the various images if you so desire.  Next, save the image as a JPEG.  Doing this will either automatically "flatten" the multi-layered image into a single-layer JPEG.  The JPEG file format cannot have more than one layer.  This JPEG is what you'd use to print the collage.  It's good practice to save a TIF and a JPEG as opposed to flattening the image and save it as a JPEG.  You don't want to do this because you won't be able to move any of the photos around.

I would recommend using Inkscape if you want to use free software. Photoshop is ideal- but expensive. Look for png images with a transparent background or use the lasso tool in photoshop or Inkscape in order to cut the characters out of the images you choose and place them on the background canvas.