First, I should mentioned Zoetrope.com. That and TriggerStreet provide peer review opportunities. After you have legally protected your work (I suggest a copyright and submitting that online -- easier and safer) you can submit to these sites and trade reviews with other writers. The systems are run on a point/exchange method and both sites are run very well.
After you have completed the short you can get feedback through swapping reviews on Zoetrope or within your film community. There are books on writing short scripts (amazon.com) and make sure you nail your genre, your character arcs, and you've written it budget appropriate. There are some who spend $25,000 and up on short films but writing to budget gives you the opportunity of shooting it yourself.
With a polished short you can then do the following:
1. Shoot it yourself. With digital video the costs come down every day. You can also rent pro-quality equipment. Sound is a particular problem area with indie films so budget for that. Most actors work inexpensively/free to get a demo reel.
2. Work with an aspiring filmmaker. You can network with them by posting your script on simplyscripts.com. InkTip allows you to post a description of your short script for free. You can search "Hoover" on simplyscripts to see several of my scripts. You can search on youtube "once bitten twilight" and see two filmmakers that turned one of my scripts into short films.
3. Contests. There are contests with categories for shorts. Many of the winners have been made. Contest win attracts attention.
Ideally your filmmaker will intend on submitting the material to film festivals. IMDB allows for shorts to be posted on the site if they have been accepted into the more prestigious festivals. Start building that list of credits.
Rates vary for the price of a short. I grant permission to use my short scripts for free but retain the copyright/ownership. That's working with student filmmakers and I'm sure those intent on pursuing it further may require some type of agreement. Make sure to consult with an entertainment attorney if possible. The main issue is what happens if your short becomes a feature film? You want to retain ownership of your characters and concept and have the short film be a springboard to a feature.
Finally, screenwriting is a career of many delays. Waiting, more waiting, and waiting to wait. Having your work turned into a film is good motivation and keeps momentum moving forward. Positive things are happening and your work is getting seen.