Missions. Yeah, it varies.
The rewards for a mission vary - the lower the security status of the system the agent is in, the better the payout. The better your standing with the agent the better the payout. The better your Negotiation Skill the better the payout.
So, focus on one corporation and on one agent in that corporation for each level - and try and find someone in a 0.6 or so system. Remember that (especially Level 4 missions) will take you anywhere in a Constellation and that could include lo-sec destinations if they are nearby.
Some missions do drop good loot. If you can salvage/loot quickly it’s a nice little side income - especially early on when you are cash short. There’s a cracking multiple Drone AI Core thingie drop in some Level 4 missions. Check prices rather than just “sell to whatever”. It might be worth hauling the tat to a trade hub. It might be worth reprocessing it or using salvage to make rigs. Ideally ones that sell!
And missions against drones and Pirates give bounty payments.
LP is useful, but do your research. The value of an object is what someone is willing to buy it for. Turnover is often as important as absolute profit. “I’m rich whenever someone buys my over priced widget that no one wants” means “I’m poor and deluded”. Fuzzworks is a great site for getting ideas about which lines to investigate.
I remember starting out, yes, I did the usual route of missions: level 1 in an Executioner then a Dragoon, 2 in an Arbitrator having struggled with a low skill Dragoon. Prophecy, Apocalypse. I used industry as a secondary source of income - it rapidly became my main source: I like the optimisation puzzle of making and selling stuff for profit. Yes, I had to save to buy an Arbitrator. It all comes with time.
Eve is a long game. Not an instant fix of happiness, but the slow burn of learning how to live and survive in a simulated world.
Take time out to do other things: don’t just grind one thing. Go exploring. Go mining. Wander into lo-sec or null and pick a fight. Chat with people in corp - even the NPC corp chat channels can be friendly. Don’t burn out in a search for ISK - ultimately it has no real world value, just enjoy living in the insanity of New Eden.
You don’t need big, expensive ships to enjoy yourself.