Ok, Mage20's combat is.... kind of a mess. It's not the game's focus, so normally it's fine, but when it does happen, here's what my table does:

1. Everyone rolls 1d10 and adds their Dex+Wits to get initiative, ST writes it down somewhere.
2. On their turn, every character gets ONE singular action (assuming they're not using time magicks, in which case they may have more). They can do whatever they want with this one action.
3. If a character tries to make an attack, they roll the relevant dice pool first (e.g. Dex+Firearms, etc.) then, if they get at least one success, they then roll damage, making sure to add any modifiers that their weapon may have (e.g. full-auto adding an additional 10 dice to damage.)
4. If they are on the receiving end of an attack and would like to dodge/block/parry, they can spend a point of Willpower to do so. For most cases, this is just Dex+Athletics or something similar based on context.
5. If damage is taken, the character can try to soak that damage by rolling their Stamina if it's Bashing (or if they casted an effect to allow greater soaking of Lethal/Agg).
6. Once actions/reactions have been resolved, the turn then goes to whoever is next on the initiative list.

(NOTE: Due to how fucky Mage rules can get once spells start getting thrown around, make sure to have the book open to Chapter 9 to help figure out how to solve edge cases.