Hi there,
You can have multiple routes with one aircraft, just make sure your aircraft has enough hours left.
Let's say you are flying from London to Paris, and want to open a route from London to Amsterdam. You are currently using 8 hours flying London-Paris-London, so your plane has 16 hours left. Let's say London-Amsterdam-London takes 8 hours too. You can do that.
But if you now want to fly to Madrid too (let's say it takes 14 hours round trip), you can't do that, because you only have 8 hours left.
In order to make money, open as many routes as possible. Having two-three short routes yields more than having one long route. Don't worry about gate costs, they are minimal.
You can find more help on starting your airline in
this video, and you can also read the wiki
here(please note that some of the information is old).
I hope this helped, if you have any more questions, please don't hesitate to ask.