"Just built my first fixie" = OP might be using a road hub rather than a track hub; if so, a (left-threaded) track lock ring wouldn't work.

If it's a road hub, you can add spacers between the sprocket and the hub flange to get a good chain line, and there should also be room on the other side of the sprocket for at least one "lock ring" (i.e., from the adjustable cup of an old-style bottom bracket).