90% of bike maintenance is tires, chains, brakes, and adjustments.
For tires: pump, tubes, patch kit, tire irons, plugs, sealant, sealant injector.
For chains: degreasing solvent, lube, rags, chain breaker, master-link plier, spare master-link.
For brakes: hex wrench set.
For adjustments: hex wrench set, pedal wrench, pliers, screwdriver.
For everything else, I would wait to buy that specific tool until you need it.
I keep my tools in a small cheap plastic toolbox.