Am not 100% sure what ABA is, so can't advise particularly on how best to 'sell' the opportunity, but however you do it, the poster should be uncluttered and tell you at a glance what its message is, then give more details etc on closer inspection. You have to balance between a big title to catch the eye of passers-by and smaller text that they can stop and read if they're interested.
Don't use Times New Roman - it was developed to fit as many words as possible into newsprint columns so isn't as clear as some other fonts. I'd avoid Arial too as I feel it's overused. And never use Comic Sans, at all, ever.
I'd do one colour on white as it's cheapest to print.