I have 5 hearts and 9 defense from my armor. My strongest weapon has 20 damage. I noticed I'm too unequipped for most stuff. I get 1 shot by any miniboss and boss bokos.
I don't have much of the map. Only the first tower and a tiny bit of depths. Any advice? Where can I get decent early-mid game loot?
The story quests should be pointing you in a couple of different directions by now. You should be equipped enough to follow either of those at this point.
The overland route specifically will net you plenty of shrines along the way to continue pumping up your hearts - there are a few caves too and those are always full of surprises.
Just be smart about what fights you take on along the way. Two things about fights in this game:
It's okay to run. If you encounter anything you aren't prepared to fight, just run. Typically you can get away on foot (or horse if you have one). If it comes down to it, warping away is also fine.
Especially in the early game, fights are trial and error. And you aren't going to find the loot you're looking for without taking on fights. You're probably going to die a few times, and that's okay.
The game doesn't punish you for dying, unless you're overly sensitive to loading screens. Try different strategies, get a feel for the advanced combat mechanics. You'll probably end up at a few different combat training shrines if you're following the early game quests and that'll help you out if you're unfamiliar with some of the more advanced techniques.
As Tolookah says, always look for a Hudson Construction Co representative at or near the tower. Often they'll have a small request for you to help the finish the tower and grant you access.
Some of the towers are puzzles to get into them. (Many?). Sometimes there's someone nearby who will tell you what they see is wrong, or to give a hint.
You should have received two quests at Lookout Landing (which I'm assuming you've been to and that is the one tower you've unlocked so far - of not, go to Lookout Landing).
One is granted by Purah and notes 4 Regional Phenomena for you to go investigate. Talking to other folks around Lookout Landing will gently nudge you toward one of the specific phenomena.
The other is granted, if I remember correctly, by Josha and involves investigating a nearby chasm to get a picture of a statue. This quest is classified as a side adventure and not a main quest, but you'll still want to do it sooner or later.
Don't overthink it. Regional Phenomena is a long quest and each investigation is a quest (at least) in itself.
As you make your way to different destinations as part of the quest, you will naturally come across shrines, caves, side quests, and enemy encounters such that you will constantly be getting rewards and upgrading things.