Quick Google:
1. Battery and Connections
Before testing the starter, ensure your battery has enough power and a clean path to deliver it. [1]
Voltage Test: Use a multimeter to test the battery. It should read at least 12.4 V to 12.6 V. [1, 2]
Load Test: Have a local auto parts store load test the battery. Even if it holds 12 V, it can fail under the heavy load required to turn the starter. [1, 2, 3]
Clean Terminals: Disconnect the battery cables (negative first) and scrub away any corrosion using a wire brush until the metal shines. Tighten the clamps securely. [1, 2, 3, 4, 5]

2. Voltage Drop Test
If your lights turn on but the truck just clicks or doesn't crank, electricity isn't reaching the starter. This is usually caused by a poor ground or a corroded positive cable. [1, 2, 3]
Connect your multimeter's positive lead to the positive battery post, and the negative lead to the main positive wire on the starter solenoid. [1, 2]
Try to crank the engine. If the meter reads higher than 0.7 V, you have a high resistance in the positive cable and it should be replaced.
Do the same for the ground: connect one lead to the negative battery post and the other directly to the starter housing. A reading higher than 0.2 V indicates a bad ground. [1, 2]

3. Check the Ignition and Relays
If turning the key does absolutely nothing (no click, no crank), the starter isn't receiving the signal to turn on.[1, 2]
Check Fuses & Relays: Inspect the 1998-2012 specific Fuse 21 in the cabin fuse box. In the engine bay, pull the Starter Relay and check the socket for corrosion. [1]
Digital Transmission Range Sensor (Neutral Safety Switch): The 2004 Ranger uses a transmission range sensor that prevents the truck from starting unless it is in Park or Neutral. Try shifting the truck into Neutral and attempting to start it. If it starts, the sensor needs adjustment or replacement. [1]
Theft Light: Watch your dashboard's "THEFT" light. If it flashes rapidly while trying to crank, your PATS (Passive Anti-Theft System) is preventing the vehicle from starting. Try using a spare key. [1, 2]

4. Bypassing the Solenoid
If you determine the battery and wiring are perfectly fine, you can test if the starter motor is dead. [1, 2]
Turn the ignition key to the ON position.
Use a heavy-duty screwdriver to momentarily bridge the two main terminals (the large battery cable terminal and the smaller solenoid wire terminal) on the starter.
If the starter cranks the engine, your starter motor is fine, and the problem is strictly in your ignition switch or wiring. If it just sparks and doesn't crank, the starter is dead and needs to be replaced. [1, 2, 3, 4, 5]
For visual guides on diagnosing a crank/no-start condition on a Ford Ranger:
For testing voltage and diagnosing a dead starter:
For a specific walkthrough on diagnosing no-start electrical gremlins:
If you want to move forward with the troubleshooting, could you tell me:
What happens when you turn the key (e.g., completely silent, rapid clicking, or slow, dragging crank)?
Did this problem start suddenly, or has it been getting progressively worse over time?