All programs are going to have their own prerequisites, there's no standard thing you need to start. Check out local prorams in your area at community colleges or hospitals and call the admissions office to see what they require.
Here is typically what you will find...
"It is strongly recommended that potential applicants begin with, at
least, a college background in algebra, biology, physics, psychology,
computer science. While these courses are not actually required,
applicants with these fundamental courses have a greater advantage due
to the point value system in the selection process."
Although the site says that you aren't required to have these college
level courses, you will have a higher chance of acceptance of being
accepted to this program if you have completed them. Usually, the
prospective students, who have the most experience or qualifications
will get priority during the admissions process. Here, in California,
you cannot even apply to RT school without completion of these college
courses. Best wishes...