More tools and supplies
You can make collages with just scissors, glue, and paper. As you make more collages, though, some additional tools come in handy:
Paintbrushes
If you're using a liquid glue rather than a gluestick, make your life easier by applying it with a small brush. The brush allows you to apply the glue in a thin layer; if the glue puddles it will wrinkle your paper.
Utility knife and ruler
You can use a utility knife and ruler to trim backgrounds to the size and composition you want.
Colored pencils
Use colored pencils to fill in detail on collage elements. Sometimes a judicious application of colored pencil can fill in missing elbows or teacup handles.
Eraser
The eraser can be used to remove your colored pencil lines. Press a little harder, and the eraser will remove the printed colors on your collage cutouts. That can be useful if you want to remove a small detail from a cutout.
Framing points and linen tape
Both of these items are used when framing. The framing points hold the collage in place within the frame. Linen tape attaches the collage to a mat board.
The full complement of tools includes a gun for framing points, linen tape, utility knife, rulers, an eraser, and colored pencils.
Tips
Archival glue
Most glues and gluesticks you buy from the office-supply store are non-archival, meaning they're a little acidic. Non-archival materials will deteriorate over time. Magazines and catalogs are also non-archival, but if you're willing to spend a little more to give your collage the best chance of lasting, buy the archival glue.
Glass
A frame looks best with glass. If you want an inexpensive piece, your local hardware store will cut windowglass to fit your frame.
