The ACT and SAT are the common tests colleges use for admissions.  Students are typically encouraged to take these exams during their sophomore or junior years.  This site will provide some free online resources students can use to familiarize themselves with these exams.

Official ACT Resources
Other (non-endorsed) SAT Resources
These sites are "use at your own risk".  Since they are not official sites, their content might differ from the standardized tests.  While I do not endorse registering at any of the links, each offers free resources that can be used to help prepare for the tests.