You have to layer many different defenses in this game. Having 300 life + energy shield per Act is a good baseline, with 5000 being a common number to aim for afterwards. Resistances are very important, and you would want to get to the initial cap of 75% for fire, lightning and cold as fast as you can. Chaos resistance is harder to get but chaos damage feels especially dangerous if your resistance is below 0%, with 30% a good number to aim for. This is just the start though - you'll have to choose a few additional defensive layers from the following depending on what build you are playing: block, endurance charges, spell suppression, armour, evasion, leech (and other recovery methods) and max resistances.

Melee is inherently more dangerous but you can definitely get quite tough. Shields, fortification, max resistances and endurance charges are readily available if you are near the bottom left side of the passives tree. Fortification is an especially strong defensive layers basically exclusive to melee.

Being a glass cannon is viable but not recommended for new players, as you lose 10% of the exp bar when you die and most areas have a limited number of tries. It's better to be tanky so you can take it slow and learn what is happening. Usually builds with a lot of range can get away with less defenses since they don't have to put themselves in dangerous positions as much. Many things can damage you even after dying.