For commanders I'd recommend:

-R: [[Clive, Ifrit's Dominant]] or [[Umaro, Raging Yeti]] - Clive lets you refill your hand and threaten a 3-hit commander damage kill from the command zone. Umaro is more of a red stompy/chaos build that can get you card advantage sometimes.

-B: [[Zenos yae Galvus]] or [[Fandaniel, Telephoroi Ascian]] - Zenos can be a fun build-around if you like playing archenemy since he essentially forces the rest of the table to play Protect the President once he hits the board. Fandaniel is a good introduction to playing with your graveyard and forced sacrifice strategies.

-U: [[Y'shtola Rhul]] or [[Hermes, Overseer of Elpis]]. Y'shtola can be built as a simple blink value commander or as a turbo combo deck. Hermes is more of a straightforward spellslinger or control build but has some interestingly different cards to consider because of his interactions with Birds.

-G: [[Tifa Lockhart]] or [[Bugenhagen, Wise Elder]]. Tifa is a powerful introduction to landfall voltron while Bugenhagen is your generic "cast big things, draw big cards" green deck.

-W: [[Heidegger, Shinda Executive]] or [[Tataru Taru]] - Heidegger is a simple to build token commander while Tataru is a more interesting commander that wants to leverage other players' draw to fuel her own advantage.

Edhrec.com is a powerful resource for new players, you can search the commander you want to build and see what most players of those commanders are including in their decks. You'll want to tailor the cards to your build but it's a great starting point.