Just follow Descartes’ logic – you perceive yourself, therefore you must exist. There are ideas in your head that you couldn’t just dream up, so they must have been put there by god (who therefore also exists), and because god is perfect (and deception is a sign of imperfection) he wouldn’t deceive you about how the world is. Therefore the world must also exist, and be pretty much the way it appears to you.

Moving on to other people, you have the evidence of words written on the internet, the most probable cause of these words is the existence of other people to write those words. That might not prove we exist, but it’s a damn sight more likely than not.

NB Descartes’ logic was full of holes and really fails to prove anything, have fun with your existential doubt.