Gnat

Deceptively simple, this model is easy to build and teaches you the basics of trimming. It is also a lot of fun.

The Gnat can be found on the  Project Air website. If you have some balsa and a prop, it can be made directly from the images on the AMA plans  site. Don Ross has written a book   Rubber Powered Model Airplanes ; an excellent introduction to free flight.   It covers basic construction and flying. 

Ajax 

Don't be afraid of building  large models; they are more tolerant of minor building errors and fly well. It can be bought as a kit kit or as a plan.

Dethermalizers (D/T)

These are devices for bringing a model down swiftly, usually by tipping the tailplane. This can prevent loss of a model or keep it away from roads/trees etc. I would wholly recommend radio D/Ts, that allow you to choose when to bring the model down. There is a whole article on types of D/T and their installation.  

Carbon Dioxide Power

CO2 engines work by supplying high pressure gas from a small tank to the top of a cylinder to drive the piston. They are difficult to regulate but have the appeal and sound of a reciprocating engine without the mess of a diesel. They are used for small models. The engines can be difficult to source.

IC Engine Power

Two stroke diesel (strictly compression ignition) engines are most commonly used. They are simpler to set up than spark ignition engines - but there is a following for those too. Fast-revving glow plug engines suit rapidly climbing power duration models. IC engines are consistent, with good power/weight ratios - but have total loss lubrication - so are messy.

Indoor/Outdoor Models

Freeflight models can be designed to fly in halls or outdoors. There is a degree of overlap, too. As there are only mild drafts to contend with, indoor models can be built very light, less than a gram for some classes. Scale models need not be waterproof and can take off from the ground - difficult to achieve outdoors. Flying outdoors allows for longer, higher flights and the thrill of chasing and finding the model again.