There is a great variety of resources to learn math online today, and at times it can be overwhelming to decide which to use.
Below are a few options that we felt confident in suggesting because they are robust and established. You will also be able to find most of these on iOS and Android App stores as mobile device apps.
Addressing online calculators:
The popularity of calculators makes it impossible to ignore their existence, so instead we would like to advise caution when using these. Use calculators as means to check your work and compliment your learning, but be warned that most professors know how to spot copy and pasted answers. Professors and tutors often watch the progression of a learner by viewing their work leading up to answers, so be careful to not treat these tools as one size fits all substitutes for your learning.
Attention mobile users if you open links from your phone, the site might realize you are on a mobile device and you will be redirected to the device's respective mobile app store. If you do not wish to download an app, then view the settings on your devices's web browser and find something that says "view desktop site," or something akin to that.
Khan Academy on its own has big variety of Math content, so I'll try to break down some of the important parts and have linked to the corresponding Khan sections below.
Developmental Math I & II covers early math which comes from Arithmetic to early Algebra.
Intermediate Algebra actually picks up partially from the end of Developmental II, so you're working primarily within Pre-Algebra and into Algebra I. In particular you will be spending a fair amount of time in factoring numbers and progress into factoring equations, then linear equations.
College Algebra focuses a lot on equations and will bounce between Algebra 1 & 2, primarily factoring, linear equations with graphing, and will build on those topics with a lot of focus on Quadratics and two variable equations. College Algebra builds up to maturing the idea of equations into the idea of functions, then exponential functions, logarithms and their uses.