You need a properly prepared roadbed, either PCB or wooden ties, and the railstock you decide to use of the appropriate code.
You would want at least one three-point track laying gauge, but it's not strictly necessary; an NMRA multi-gauge works okay. It's just easier with two of the three-point ones.
Scale spikes, and a tool to insert them into the ties if you are using wooden ones.
Soldering iron and supplies for using PCB ties. Probably 30 Watts should do.
A metal flat file, commonly called a bastard file (if there isn't a censor on the software here).
Xuron rail nippers.
Needle files for metals if you are making rails with the PCB ties. They'll each need a gap in the copper cladding between the rails, or else you'll short the system across the expanse of copper between them.
A very handy tool if you are making turnouts is a jeweler's saw. You must gap the frog to prevent a short at the frog point. The thin filamental blade cuts a very thin gap that is close to invisible.
-Crandell