search "essential EDH cards" and for a list of the most commonly played Commander cards. There's also a site where you can see what the most popular cards for every Commander is, to get some ideas
For starters, consider getting Mimic Vat because it's a cheap card, goes into everything, very good and it's also just hilarious. Isochron Scepter, because chaining Condemn or just Ponder every turn is fun and incredibly annoying
since you're playing tribal, you'll absolutely want Door of Destinies and Coat of Arms, if they're aren't in the base deck already. Cauldron of Souls, Selfless Spirit, if you want your creature-heavy decks to survive a board wipe or too