It could be the starter but it could be the battery, how old is it?
Get the battery tested first (99% of automotive places do this for free), just because the "indicator" says it's ok doesn't mean a thing, for a start that thing is only in 1 cell of 6..... a battery doesn't die across all cells at once, never rely on that crappy little indicator, you need to test all 6 cells with a hydrometer not just 1 to be able to say what state the battery is in but these maintenance free batteries don't allow access (unless you smash the top off the battery and render it useless....) so a load tester is the next option.
In addition to the instructions for jump starting a car, why you should always connect the final negative clamp to the engine or chassis and why you always disconnect it first is it creates a spark.
If you have been cranking a battery for ages and run it flat or it has been charging, a lead acid battery it will have generated hydrogen gas, sparking near this is dangerous and can result in the battery exploding and sending acid all over the place including you (it's a low possibility that this can happen, it's better to be safe than burnt and blind, stupidity taught me long ago not to create a spark near a charging battery..... wrecked my good shirt taking a shortcut and blew the top off the battery)