* The ignition switch cylinder (the part where you put the key)? This is usually removed by inserting a special tool (sometimes called a "ignition switch removal tool" or a similar description) into a small hole visible when the key is removed. This tool depresses a retaining pin, allowing the cylinder to be pulled out. You'll need to find the correct tool for your specific Astro van's ignition switch. Auto parts stores sell these or you may need to consult a repair manual.
* The ignition switch itself (the entire electrical assembly)? This is a much more involved process requiring significant mechanical knowledge and likely specialized tools. It's usually accessed by removing the steering column shroud and other components. This is *not* a job for a beginner and a repair manual specific to your 1998 Astro van is absolutely essential.
* The ignition coil (part of the ignition system)? This is a component under the hood and is removed differently again. It's usually held in place by bolts and wires. Again, a repair manual will show you precisely how.
* Something else entirely? Please clarify what part of the "ignition" you're trying to remove.
In short: Before attempting any of these, consult a repair manual specific to your 1998 Chevrolet Astro van. Trying to remove internal parts of the steering column or ignition system without the proper knowledge and tools can cause significant damage. If you are not comfortable working on your vehicle, it is best to take it to a qualified mechanic.
