First you need photos. Good photography is important!
Next you need to print the images or make a compcard.
If you don't have a modeling compcard you can make one here
Nice headshot, nice smile, nice energy shot, ( could be laughing,dancing, or running, and shot that looks editorial, with a product or could be your jeans shot. (to show diversity). Or IT IS OK, to print out your photos and mail seperate photos to an agency. You wont find petite modeling agencies bluntly. You will find print modeling agencies, or commercial print modeling agencies.
I think it is always good to self serve and to look for your own modeling work to grow your modeling portfolio and resume. If you can go to an agency, or mail an agency with already a tearsheet or some modeling experience it will make you stand out.
An idea is to mail your photos to aspiring brands, or smaller magazines or newspapers in your hometown. Go to events and tradeshows, even craft shows, and try to meet aspriing brands for who you could model for. Bring some photos or compcard and try to book your own modeling work. Of course there is Craigslist.com but you have to be careful.
I will have a post soon about "HOW TO MANAGE Craigslist as a MODEL," which you might want to check out. There is also Mandy.com and other websites like:
And Backstage.com have modeling work too. And you can find a list of modeling agencies and talent agencies too.
I would google: modeling jobs
modeling print work
But I would always be VERY careful when searching online and think through every submission, because there are ALOT of scams out there.
Also I wouldn't be afraid to print up some of your photos, (a good source is http://www.adorama.com/)
Then with your printed images, write a cover letter stating the type of modeling work you would be good for: Commercial work, fitness, swimwear, etc and don't forget to sell your personality and any hobbies you have and your interests. Show you are driven and capable.
I hope this helps!