• Read this point of the FAQ, especially first bullet point. It's probably the only one which applies to your situation
• Read through posts below. Unfortunately, many of them is marked as spoilers, but you might be able to find something useful.
• Do some more research. Google, read, work with pen and paper. Try to find out how far you have to loop to be able to determine that number is prime. Hint: it's not n, nor n/2, nor n/4, nor...
• It depends on language version, but in most of them, numbers go up to 2_000_000_000. It might seem scary, but it's easier than one would expect.

I plan on adding more information on range of tested numbers to the description, so it would be known from beginning.