Track width and clearance can be set for each net from PcbNew from the Design Rules -> Design Rules menu, Net Classes Editor tab. Those values are used for both design rule checking and autorouting.
You can set up PcbNew for a single layer board from the Design Rules -> Design Rules menu, Layer Setup tab. That should force the autorouter to try to do a single-sided board. But don't expect a successful one-sided design for anything non-trivial.
Why are you doing a one-sided board, anyway? Everybody uses at least two layers today. Most board houses have their cheapest offering as two-sided with plated-through holes.