Yes, onto is a real English word, a preposition. Use it when something moves to a position on something, like a surface, platform, or path: The cat jumped onto the sofa. It can also be written as two words, on to.
If you're describing a spill, The coffee slid onto the desk works because the coffee ends up on the desk. When your sentence describes only a position, with nothing moving, plain on is the natural choice: The cat slept on the sofa. Reserve onto for sentences where something moves to that spot.
If something ends up on a surface, onto shows the movement that gets it there.
Spilling coffee: The coffee slid onto the desk. The liquid finishes on the desk's surface, which is what your reader pictures.
A step class: She stepped onto the platform. Her foot lands on it, and the word marks that landing point.
Moving a file in an app: Drag the icon onto the folder. The icon ends up on the folder image, so the same rule works on screen.
Stage directions: He climbed onto the roof. The sentence tells the reader where he ends up, on top of the roof.
Contrast: The cat slept on the sofa. Here nothing moves, so plain on is the natural choice, and onto would sound odd.