Look for a bread flour with 12–13% protein. That's the strength you need for the gluten structure sourdough demands.
Freshly milled matters more than brand. Flour begins losing flavor and enzymatic activity the day it's ground. If you can source from a local mill, do.
A small percentage of whole wheat — 10–20% — feeds your starter and adds flavor without weighing the dough down. All whole wheat is a different bread; delicious, but denser.
For beginners: start with the bread flour at your grocery store. Get comfortable with technique before you chase specialty flours. The bag isn't holding you back.