Use Photoshop or some free equivalent, like the GIMP.
Look at other event posters and copy their design if you don't have an idea of your own.
Other than that, make sure all the relevant information is on the poster and is legible.
The Alt Alphabet ~ OPC: Other People's Characters ~ Terrific Screenshots of Cool ~ Superhero Fiction
Here is a site that is set up to make a mock magazine cover, but you may be able to do something like what you want there.
My Deviant Art page link-link
CoH/V Fan Videos
Thank you guys 
@.MIDAS
BIG BROTHER MIDAS, Ice/Ice Dominator, 1314 Badges
* Any toon you see with Midas in the name is probably me
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Hello all, I've never made a poster for an event and I'm looking for some input on which program to use and the process. Can anyone lend a hand?
@.MIDAS
BIG BROTHER MIDAS, Ice/Ice Dominator, 1314 Badges
* Any toon you see with Midas in the name is probably me