PLA (Polylactic Acid) is probably the easiest material to work with when you're first starting out with 3D printing. PLA is a biodegradable thermoplastic that has been derived from renewable resources like corn starch and sugar cane. This makes PLA environmentally friendly and very safe to work with.
PLA also has a very sharp glass transition point so if you use a fan to cool it, it will set into a solid very quickly. PLA can achieve a greater range of geometries than are possible with other plastics. It also reduces the thermal stress on the printed part, making warping less of an issue with larger PLA parts.
PLA does not require any curing or post-production treatment. Should you wish to, however, PLA can be sanded and coated with automotive spray filler. PLA can also be painted over with acrylic paint.