Hi Adil,
 
Public elementary schools in the United States are typically free to attend, and have age (and immunization) requirements for starting kindergarten.
 
To help you locate a school for your daughter, here is a listing of Portland, OR schools in SchoolFinder: http://bit.ly/4RIqO  Given that Portland is so large, I would narrow the search a bit further by entering your zip code into the search field on the SchoolFinder homepage: http://www.education.com/schoolfinder/  Then, under the map, you will see school district links for your zip code. Click on each district's link to find their contact info, and then call them (or check their website) to find out which school your child is zoned for (and/or to learn the district's policies with regard to enrolling your child in elementary school).
 
Should your child need to attend preschool this year (and start kindergarten in 2010 when she's 5), you may want to consider the Head Start program for low-income families. Here's a fact sheet about Head Start from the National Head Start Association:
  
http://www.nhsa.org/files/static_page_files/A97D606C-1D09-3519-AD7E2646D5EA7CDB/08FactSheetHSBasics.pdf
  
And here is the Head Start locator on the U.S. Department of Health and Human Services website, so you can find a center near you:
http://eclkc.ohs.acf.hhs.gov/hslc/HeadStartOffices
  
If your family is not qualified to participate in the Head Start program, some preschools offer scholarships. You can contact potential preschools in your area and ask about their financial assistance offerings, or here's a video about scholarship money for preschool: http://www.ehow.com/video_4790646_free-scholarship-money-preschool.html
 
Below are some additional free resources you may also find helpful in this stage of your child's education and development.