Just starting

resistance is key to survival... Each act will lower your elemental resistance. so make sure you are upgrading your gear to compensate

try to keep your flasks up to date as you go. (long time player who always forgets this)

Read EVERYTHING. I'm not kidding. Every skill, every item, every message that appears is important to read. In the skills, read all of the lines, including the stuff you would normally ignore. Click on the right-arrows on your skills in your skill gem window, and then look through all the tabs.

For spells, you want spell damage rolls on your items. For attacks, you want added physical damage primarily, and then added damage of other elements.

Added damage is the best, followed my "X% More", and then "X% Increased". Added levels to skills can be very strong, but can

Resists are best on your rings and amulet. Life should be on as much gear as possible.

Take the time to learn the basics of the skill tree, or lookup build guides online if you feel overwhelmed.

On top of all the stuff mentioned here, download a loot filter.

Beyond that, enjoy your time!

Gear matters a lot more , especially always upgrade your weapon and skill gem whenever you feel lacking damage. Rest cap your resistance and get good armour

The orbs that turn normal gear into magic gear, or adds a 2nd affix to a 1 affix magic gear, or upgrades a magic to a rare- none of these are valuable for currency, so feel free to use them liberally while trying to find better gear. Essences of a tier below 'greater' (lesser and normal, basically) are also worthless at higher levels, use them liberally. Exalted orbs are a notch above the rest in terms of how much value they have for trading.

Trading for better gear typically is the faster path of power progression until you're a more experienced player.

Any class can use any weapon/skill. So you could try spears, bows, crossbows, or even the shapeshifting talismans on your warrior if you wanted.

Have fun, you are going to enjoy campaign!