Pre-Algebra Vs Getting ready for Algebra 1 Vs Algebra basics

Each of these courses has a Course Challenge button on its main page.  Try taking the course challenge for each.  Then, tackle them in order from one for which you have the most Mastery to the one for which you have the least.  Many of the skills are repeated across courses.  If taking the Course Challenges gives you a high enough level of Mastery in Algebra I, you could just start with that.