How do I know which size shoe to pick?

A good rule of thumb is that there be about the width of your thumb’s length between the top of your big toe to the end of the shoe

When purchasing a shoe, what are the main things I should look for?

Quality of workmanship. For example, an improperly placed tongue will irritate the foot and cause discomfort when will walking around in them

How do a read a sizing chart? For example, what is a 7B?