It depends on where you need to align to, since polylines can be multi segmented objects.

The easiest thing to do is use the rotate command. When you start the command, it’ll ask to select your text, then it will ask for a base point. Select a point along your polyline, an itnersection or endpoint will be the easiest point to start from. The next step requires attention to the command line. There is a Reference option. Select that and it will then request a reference angle. From here, you need to select a base point, a second point on the same axis as your text, and then a final point along your new axis. For example, if your test is horizontal to the screen, and your line is not, you would select your reference angle base point as a point on the line, the second point would be directly to the right (turn on ortho mode to get a true angle) and then the third point would be farther up your line.