Speaking a little about MM6-8 and how they relate to the HOMM series, in 6 you are playing in Enroth after the succession wars of HOMM2. Roland is your king, and Archibald is imprisoned in a state of petrification that can't be easily undone. You can walk into the very room he is in early in the game, and as you can see, this is actually a great deal of involvement with the HOMM storyline. In the land, the devils of Eeofol, which are unveiled in HOMM3 as the Inferno side, have invaded Enroth for the very first time, and the overall quest in the game is to help Roland save the world from them. You do this by taking a small party of four characters into dungeons and harsh wildernesses, piecing together whatever information there is available for how to fight the threat, which has the military completely engaged in like a stand-still at the front. You actually came from a village out there, and were saved by your mentor, a wizard who teleported you to the opposite corner of the map where you start play. The hope is that you'll be able to do something, and earn experience to become strong enough to face the devils.
MM6 is definitely the game to start with. The user interface is not difficult to learn, and as you begin just try to only take on a few monsters at a time as there are some very large groups that can surround you if you get too close. In addition, just try to take one quest at a time and work on gaining levels and training your characters' main skills. The game suggests your party be paladin, archer, cleric, and sorcerer. If you take this, advance the following three skills first for each character: sword, shield, and repair item for the paladin, axe, bow, and disarm trap for the archer, body magic, spirit magic, and merchant for the cleric, and fire magic, air magic, and identify item for the sorcerer. These are just some frequently chosen combinations, but anyone can get the utility skills of repair item, disarm trap, merchant, and identify item.
Good luck and keep us informed of your progress!