Objectives vary depending on which mode is played, but the broad goal of Fortnite is to survive against a prolonged force of antagonists who attempt to eliminate you. The "antagonists" are other players and Henchmen in Battle Royale and Battle Lab (Matchmaking), and otherworldly hostile monsters in Save the World. Creative features no enemies, except player-created ones.
1: Unlike most other games, as a general rule you should avoid combat as much as possible. Only fight when you need to or when it serves some quest/mission objective. The reason for this is that all combat (outside of special abilities) consumes ammo and/or weapon durability while only providing a tiny amount of experience. Fighting is generally a waste of resources. Pick your battles.
2: Almost everything has a level and quality. Your weapon, hero, survivors, etc all have quality levels and experience levels. Higher quality items are generally better than lower quality ones and will benefit from additional levels more.
3: Avoid spending experience on anything less than a rare (blue) quality item. Unless hard up on useful items, don’t evolve anything less than a rare, and only evolve a rare once. Hold out for better.
4: Focus your schematic/hero experience on a narrow set of options. A couple really good choices are better than a lot of so-so ones.
5: Unlike schematics & heroes, spread your survivor experience around. Having a bunch of survivors with a couple of levels each is a lot better and cheaper than getting one with a lot of levels.
6: Squads are important. Read up on how squads work.
7: Be courteous to other players. If you want to just harvest resources, or try one of those red missions to test your mettle, create a private mission. Additionally, use missions without a time limit for completing side quests, unless completing the mission helps complete the side quest. Otherwise, you will be taking up a slot that could be filled by a player actually helping.
8: Pay to wait.