1x4 wood similar to first one found here: https://www.ag.ndsu.edu/extension-aben/buildingplans/beef
Or quickest, go buy a cheap metal tube gate from tractor supply (or other). Then bolt pickets to it. http://www.tractorsupply.com/tsc/product/utility-tube-gate-50-in-h-x-12-ft-l?cm_vc=-10005