The best bang-for-buck habit would be a solid morning ritual/routine. Have 4-5 things that you line up every morning and can successfully execute and that will get you in the right frame of mind and performance state to have a really productive day.

Discipline is something you build more over time and takes both successfully building habits and staying away from distractions before one day you wake up and realise "oh wait, I'm disciplined now".

As others have mentioned, behaviours like planning your day the night before are also extremely effective.