Buying brushes is unnecessarily complicated and expensive. When first starting out, it's helpful to get brushes that can be used for multiple steps. There are many ways you can build your own set, but for starters, beauty vlogger Emily Quack recommends the following budget-friendly makeup brushes:
Beauty Blender (dupe) ($7.97): Not technically a brush, but it gives great coverage for foundation and can also be used to blend all makeup from blush to concealer.
Angled Brow Brush ($8.75): Good for eyebrows and smudging eyeshadow under and in the creases of eyes.
Sigma Blending Brush ($15.20): Great for blending all types of eyeshadow and for nose contouring.
Sigma Large Angled Contour ($23): Amazing for contouring and blush application.
Multitasking Brush ($4.49): Great for applying powder after contouring and blush.
If you want to know more about how to use these brushes, check out this detailed explanation here.