How do you balance encounters for a party without team composition?

Prepping for a Homebrew campaign for a party of mostly new players.

As a DM for inexperienced players, I find the freedom of character creation to be the best way to have the players hooked to D&D. However, the players have not yet understood why balancing their party could be very important.

Take in mind that for most of them, this will be their first chatacter for their first actual campaign. I want it to be as memorable for them as possible.

As of now, no one seems to want to play a proper tank or even a healer of some kind.

Have you experienced handling a party like this? What adjustments did you make for your encounters? What items or magic items did you give your players to make up for some of their weaknesses?