This is my first time using this and coming from flying commercial jets with FMC and all, I'm trying to figure out how to create flight plans. Can you make a plan from scratch without having to pick a STAR/SID? Seems like I can't get by this.

General aviation ie small props and twins don't normally fly stars and sids, at least not the jet sids/stars.  ATC usually vectors you out around their busy airspace and gets you on your filed plan as soon as they can.  But sure you can make a plan from scratch.  FPL button; scroll down to the entry below your airport; scroll with the smaller knob and "spell" out your waypoint or airport.  Larger knob moves to the next letter position.  Alternatively you can use the direct to button to your destination airport and then fill in the waypoints between.

Enter your departure airport using ICAO format (4 letter). If you need to fly a DP highlight the departure airport and press PROC button. Select departure then continue adding waypoints. Complete FP by adding arrival airport. If you need a star do it the same way: highlight arrival airport and press PROC select arrival.

 

You can also activate legs of the flight plan , highlight the fix and press MENU: activate leg. Handy if you want to intercept.