to lock those planes, highlight one of them (select it) and then create a new layer on the right in the channel box, right click the newly created layer and add selection to layer. You can rename the layer so you know what it is. Then do it with the rest of your reference images.
To lock them there will be 3 boxes next to the layer name, click the middle box until there is an "R" and the image should now be locked and you can't select it or move it.

to work on one side of the model I believe he means front and side views. Hit the space bar and it should go into top, front, side, and persp views. Put your mouse over which view you want to work in and hit the space bar again to bring yourself into that view.

Hope that helps!