Any tips for commander deck building?

I've been trying to get into commander but every game I play my opponent always seems to have every counter they need in hand at all times, always has mana to spend it, all the while also summoning their commander/heavy hitting creatures. I keep adding cards that I see other people use to make me draw more/get more mana but it just never works for me like it does for my opponents. And the games where I'm doing ok and might win they suddenly pop out an infinite mana/damage combo and 1 shot me. I have not won a single match of commander. Any tips?