First you've got to find out what the problem is, so let's do that.
Look up under the ignition and remove the screws holding the ignition housing assembly from the support plate holding it to the dashboard. You may also need to remove fasteners holding a side support bar, then pull the steering wheel back towards you to move the housing off that plate.
Then look up underneath the housing assembly with a torch and locate the screw hole, roughly in the middle- surprise surprise, near the switch. Remove that grub screw and pull the ignition switch off and try the key again.
That'll either solve your steering lock problem and allow you to hotwire the van (if the issue is a broken switch w/parts obstructing movement) or narrow it down to a new ignition barrel/removing wafers from your old one if you're cheap and don't tell your insurance company.
Assuming the latter is true, you'll then have to remove your ignition housing assembly to get the barrel out. horn push gets removed, undo the screws and bits and bobs under there, and loosen (do not remove fully just yet unless you want to smash your face off on removal) the main steering wheel nut. You may have to apply some boiling water to the steering wheel hub to expand the joint and allow it to loosen properly, but you should be able to get the wheel off without a puller. Remove the indicator switch, should be 2 teeny allen headed screws near the stalk. Keep them, they have an unusual head OD.
After that, you should pretty much be able to get the ignition housing out. Do so, then follow these instructions, make sure there are no kids and maybe some telly/beer as it's a little fiddly.
http://www.specialpatrolgroup.co.uk/spooky/barrel/barrel.html