walkthrough on creating a classified site

I am trying to create a niche marketplace where people can buy and sell used bikes.

I have installed all the plugins, and I am having just a little bit of trouble getting off the ground.

I need website visitors to have the ability to create an account and post from the front end. I have tried a few of the shortcode combinations, but I think I am doing something wrong. Can you please offer some guidance?

My url is niceusedbikes.com

Thanks!

jc