If you're looking for a simple start, I would recommend starting with either
Mega Kangaskhan or
Mega Blaziken as they're both two of the most simple and effective Megas to use in BSS. It's hard to give advice when you have little to work off of but it's definitely more fun to plan your own team if you start with a favorite that's also reasonably effective in the format. With that, Volcarona,
Hippowdon and Cloyster are all fine choices in BSS. Volcarona often works as a Quiver Dance sweeper though it needs support to handle Talonflame and Fire-types in general, and if it chooses not to run HP Ice then something to handle Dragons like Salamence. Cloyster functions as a Shell Smash sweeper though naturally has to be careful with stuff like Thundurus and Suicune. Hippowdon is commonly used on cores with Mega Lucario and a dragon (often Dragonite) so that's a core you could start with if you're looking to go that direction. Either way it's tough to give specific help until you've decided what you want to run, but hopefully this gives you some ideas.
