You indent line A with tabs. You use tabs to line the start of line B up with the start of line A (indentation) and then use spaces to move it the rest of the way to where you want it (alignment).
And as an aside, it's worth pointing out that most editors will handle the initial tab part automatically, so it's not like you have to do mental math to think "okay, this many tabs, and now switch to spaces". Just at the end of line A, you hit enter and start hammering the space bar.
