If you have WH 1 and 2 on Steam it should allow you to choose the Mortal Empires map when you go to start a new game in WH2.
If that's the case, then yea, Mortal Empires is probably the easiest to jump into. Just try to follow the objectives for short and long game wins.
Lizardmen, High Elves, and Dwarves(maybe? see below) are going to be the easiest races on ME. Any lord should work, but a few Legendary Lords are super strong (like Mazdamundi for caster and Tyrion for melee god).
Dwarves are well rounded and good at standing their ground, but Orks were recently overhauled. So, the Dwarf start might be a tough neighborhood these days. Also, Lizardmen and High Elves get more interesting mechanics, since they are WH2 races.