I would still advise reading a basic 5e combat summary - it's quite different from some older D&D versions (as well as how things like rogue's sneak attack works - not just being behind someone for example).
https://roll20.net/compendium/dnd5e/Combat#content[NOTE: Larian have changed quite a lot, but the basics will seem much more familiar - that site has descriptions fr most classes and abilities too.]
Also attacks of opportunity as Buba mentioned - when you engage an enemy in melee, you need to be careful how you move in combat, else they will whack you.
