Even if you’re all in I still recommend you start with a RTF heli, but if you want to go straight to building your first model being aware of the following points will ensure you know what to look for and avoid finding yourself in a place of regret.

• Model sizes are almost always determined by the length of the main blades in millimeters. 700+ is considered large, 400-600 is consdered medium and anything under that is classed as Small or Micro which the RTF models typically are.
• A larger heli (typically) is more stable and easier to fly, however the bigger and heavier they are the more damage they will sustain in a crash making repairs all the more costly. 
• The part of the helicopter you see in the air is called the Airframe.  Always choose a model that is well supported for spare parts, as having a brand that is ‘stranded’ may leave you with a heli after a crash that will never see the sky again. The components that complete the airframe into a flying model (like a motor and servos) you choose should be renowned for quality and support.  Any of the products offered by the businesses listed on this site only stock items that fit this description, but you should always do you own due diligence especially when buying used.  
• The flybarless (FBL) controller is the one component that requires the most finesse and attention to detail. Get the installation or setup of this wrong and you’re almost guaranteed to cause significant damage without leaving the ground, add to the fact it becomes very dangerous as it will not respond correctly to the commands you give it. Choose a FBL that someone at your local club uses who is also willing to help and coach you.  I personally started with one brand using this approach and then moved to another brand I preferred later once I had a firm understanding.