Hello, best to start at the top before guessing at a starter motor concern. Additionally, if the alternator is at fault, you'll still need to start at the beginning of the diagnostic flowchart.
You always will need to check the 12 volt engine battery to see if it is fully charged and able to sustain the load of the vehicle and starter motor. Once this is checked and verified to be good and strong, then you'll want to check all connections of the cabling. Are they all secure? Free of corrosion?
After all that checks good, if the same symptom remains, then you'll want to check for excessive draw from the starter motor. If present, it will need replaced - then recheck all.
If the battery was dead, but comes back to a full, good charge - and the vehicle starts -- then you'll want to check the charging system, including the alternator, to ensure there is output sufficient to recharge the battery while the vehicle is running.
Beyond all that, and additionally to all that, you may need further assistance and testing.
Use this link to find some shops near you that can help with your concern:
www.repairpal.com/repair-shops
