99 Jeep Wrangler won’t start

First, test how much juice is left in the battery. The starter may be totally fine, but a battery with low-charge won't cause it to start. If its low, get a battery charger hooked up to your battery, and let it charge for at least 15 minutes to get some meaningful juice in it. Next, if that doesn't work, it may be the Starter. Have you ever had the Starter replaced in the past? If the relay clicks but the starter doesn't crank at all, then your battery and fuses are probably fine, and the Starter may be done. If you replace the starter and the battery & fuses are good, its time to look at your ignition coils and spark plugs. Bad or broken ignition coils will have a white color on the bottom tips where they insert over the spark plug. Replace any bad spark plugs (black / rusted out) as well!