Car been sitting for a while. What could my problem be?

The clicking sound means you don't have enough power to turn the engine over. The fact that some things work just means your battery isn't totally dead. Not sure how your AC is working without the engine running...

Just remove the battery and put it on charge. Or use jump leads. Or tow start it. Oh, better not. You're probably in America and chances are haven't learned how to coordinate a hand on a gearstick with your foot on a clutch so you can't tow start it. lol

Remove battery terminals &, clean the mating surfaces & replace them; charge the battery overnight (preferably with a modern smart charger) then try it again.

Quick check of terminal condition & battery. Headlights on low, headlamp flasher pulled, foot on brake. Does the horn still sound normal? (ie load up the battery with as much as you can & see how well it performs.

If it passes this test next most likely culprit is the starter solenoid.

Charge the battery...just because you have lights and the blower motor still works doesn't mean the battery is fully charged!

It takes 6 hours with a charger set to 12 volts, 10 amps to fully charge a battery.

Get a jump.  Truck starts, charge the battery.  Truck does not start, get it towed.  Starter could be shot.

It is the battery. Jumpstart the truck and drive it for an hour, or charge the battery overnight.

The battery is dead

Think of it this way. It takes a small percentage of a battery's power to light up any of the car's electrical accessories. To start the engine all power is intentionally routed to the starter. Why? Because it will require most, if not all the battery power it has, to spin that engine. 

Your battery has depleted below the power level to spin that engine. Recharge the battery and don't wait so long to restart your truck next time. You may have a parasitic draw in your truck. 

Charge battery and try again - no need to disconnect battery - apparently some  people don't understand how diodes work and that the battery is being charged hard when the car is running. Lights, radio use very little amperage, so they work fine on a depleted (partially depleted) battery, the starter takes many more amps and the battery isn't up to the challenge at the moment (from sitting)

Being a 1997 it should have a large square starter solenoid right there by the battery did you should be able to jump I would suggest you charge the battery first and then take your jumper cable and jump that starter solenoid. Turn your ignition into the next to the start position. Jump the solenoid and see if it starts if it still don't turn over then you know what's wrong your starter is gone.

Flat battery. Disconnect it or take it out and charge it.  Never charge with the terminals connected. Even new batteries can struggle to hold a good charge for a month. I have started charging my own two cars each month during this lockdown.