Thought process/ hand ranges

What kind of things should I be asking myself on the flop. I can see that preflop I should be looking at who enters the pot and from what position they came, but what about when the flop comes on the board. I know I need read the board and count my outs, but what else should I be thinking about. Let's say I'm last to act and my opponent bets at the pot, how do I know what that bet means.
Also, how do I put my opponent on a range of hands? I read about it all the time, but what is the thought process on putting my opponent on a hand or a range of hands?

You develop opponents' ranges from the info you obtain by watching them. For example, how often does player A enter pots? How much does this differ according to position? How often does he raise vs limp or call? How often does he try to steal? If he has shown down suited connectors, how did he play them? How about small and medium pairs? Etc., etc.

It's also helpful to familiarize yourself with what hands fall into various ranges so that, for example, if someone is opening 15% of hands, you have a decent idea of what's in and not in that range.

On the flop, one thing to look at is how well or poorly it fits the range you think he has based on his pre-flop action.