One common problem is corroded cables where it's hidden under the ends by the insulation, particularly down by the starter.
Start by measuring the battery voltage. It should be 12.6 volts. If you find closer to 12.0 volts, it is good but discharged. We'll have to check the charging system with the engine running. At 11.0 volts or less there is likely a shorted cell and the battery must be replaced.
If the battery is fully charged, you'll need a test light and a helper to turn the ignition switch. You'll be looking for the first place down the line where voltage is lost when trying to crank the engine. I can help with those steps.