I know about opportunity attacks, but what about reach? I don't know how to find out what my reach is.
The answer can be found on page 195 of the PHB:
Most creatures have a 5-foot reach and can thus attack targets within 5 feet of them when making a melee attack. Certain creatures (typically those larger than Medium) have melee attacks with a greater reach than 5 feet, as noted in their descriptions.
Aside from that, certain weapons also increase your reach if they have the reach property, as described on page 147 of the PHB:
Reach. This weapon adds 5 feet to your reach when you attack with it.