startup, upstart

Startup or startup ( one word ) means a new company, one that has just begun operating--
Their startup is so successful that they plan to go public in a few months.
He hopes to be able to make a living from his startup after 6 months.

Start up or start up ( two words, sometimes start-up ) means to set something in motion, as when starting a machine or motor, literally or figuratively--
He has to go in early to start up the engines.
Start up may also mean the initial motion or operation of something, as a machine--
The machine always makes that noise at start up.

Start up may also mean to jump up in surprise, or to begin to move very quickly--
The loud noise made him start up from his chair in surprise.

Upstart or upstart means a presumptuous person, one who has risen from nowhere to a position of prominence--
Does anyone know where this little upstart came from?
They all resented the precocious upstart who seemed to have succeeded so easily.

Now that you know that, you can say--
"The internet upstart was sure that his startup would be very successful."