The simple truth is getting your resistances sorted early makes surviving way easier. You'll want to aim for that 75% cap on fire cold and lightning resist, since going through the acts hits you with some nasty penalties that can drop you down by 60% total. The good news is getting there isn't too complicated once you know what to look for.

Start by checking vendors for ruby sapphire and topaz rings, they're great early game and you can usually find decent ones with extra resists as bonuses. Your jewelry slots are actually super important here since rings and amulets can roll pretty high resistance values. For armor pieces try to get at least two different resistances on each, and don't worry too much if you need to sacrifice a bit of damage early on to stay capped.

Once you hit maps, things get a bit trickier since you'll need even more resistance to stay capped. Chaos resistance becomes more important too, though you can usually get by with just keeping it above zero. Something interesting to keep in mind is that having good resistances also helps against those nasty elemental ailments like shock and freeze, which can really mess up your day if you're not prepared.

A solid tip is to use your crafting bench to patch up any resistance holes in your gear, and maybe grab some resistance nodes on the passive tree early on that you can respec out of later. If you're really struggling, purity of elements is always there as a backup plan though it does take up a socket and some mana.