Most MMOs make you stronger with gear. City of Heroes doesn't have gear: your powers are your gear. You make them stronger by putting enhancements into their slots. How you slot comes down to how you want to play, so there's no single right answer, but some advice holds for every hero. Here it is, straight from Haven's staff.
Your first build will be a mess
Expect it. Almost nobody gets their first hero right. Powers turn out weaker than they sounded, you run out of endurance, you put slots in the wrong places. That's normal, and it's how everyone learns this game.
The good news: you can always respec and rebuild (Chapter 7). Nothing you pick while leveling is permanent.
The basics
Every new power comes with one slot. On the levels where you don't get a new power, you get extra slots to place wherever you like: 2 at a time early on, 3 at a time from level 31. Levels 41, 44 and 47 give you 3 slots on top of the new power, and level 50 gives 6. A power can hold up to 6 slots.
Right power only
An enhancement only fits a power that uses it. Defense boosts go in defense powers, Damage goes in attacks.
That power only
It only boosts the power it's in. An Accuracy in one attack does nothing for the others, so every attack needs its own.
Level window
You can slot enhancements up to 3 levels above you. Store-bought ones stop working once you're more than 3 levels past them.
Crafted never expire
Crafted IOs keep working forever, at any level below you (Chapter 4).
Slot your real powers first
Your slots should go to the powers that actually win fights: the attacks you use most, your armors and toggles, your heals.
Attacks
Heals
Toggles
Upgrade your enhancements
Each step up is a big jump in power:
Training
Drop from level 1. Anyone can use them. Weakest, but fine to start.
Dual Origin
Show up in the teens. Each fits two origins (the one you picked at creation), so check the label.
Single Origin
Show up in the 20s and fit one origin. Use the map to find your origin's store. Pricey at first, worth it.
Crafted IOs
Made from recipes and salvage. Strength depends on their own level, so they never fade as you level.
Crafted single IOs (one stat each, like plain Damage or Accuracy) beat Single Origins once they're high level, roughly from the 40s, and level 50 ones are the best. They also never wear out or turn red.
Set IOs can have smaller numbers per piece, but they add set bonuses (Chapter 6). In the long run those bonuses can help your build in ways you didn't plan for at first.
Heads up: run a difficulty that's comfortable for you and that your current enhancements can handle. IOs a few levels behind you won't keep up with enemies 2 or 3 levels above you.
Two store-bought tricks
Combine: click a slotted enhancement and merge an identical one into it to raise its level by one (up to twice: + and ++). The bigger the level gap, the higher the chance it fails. Hamidon enhancements are rare drops that boost two things at once.
Save these for last
Don't put extra slots into Health, Stamina or Sprint while your real powers still need them. Only slot these when you have slots left over and nothing else you want to max out. When you get there, they're great homes for set pieces:
Health
Takes healing IO sets.
Stamina
Takes endurance IO sets.
Sprint
Takes running and universal travel IO sets.
Sets & their bonuses
Set IOs come in sets for every kind of power: melee attacks, ranged attacks, heals, holds, slows, defense and more. One set piece often boosts two, three or even four things at once. Put two or more pieces of the same set in one power and you unlock set bonuses that help your whole hero: extra defense, recharge, endurance, health, damage.
So pay attention to the sets you're using and the bonuses they give. Read the bonus list before you buy, and pick sets that push your hero where it needs to go. Stack enough recharge, for example, and powers like Hasten can stay up all the time.
A few special pieces give a bonus all by themselves, like a resistance piece that adds defense to everything, or a defense piece that adds global recharge.
Planning a build? The IO Set Finder lists every set and its bonuses, lets you find every set that gives a bonus like Recharge (best first), and shows which set types Haven's custom powers take.
Haven makes this easier
Every recipe and salvage is on the auction house, so you can craft your own sets. Level 48+ enemies drop Reward Merits for set IOs, plus Converters and Unslotters to fix a build without starting over.
How a respec works
While you level, you alternate: a new power one level, new slots the next, back and forth. A respec is different, and it surprises people the first time:
Pick every power
You choose all your powers first, level by level, before any slots.
Then every slot
Only after the powers are done do you place all your slots.
Then enhancements
Finally you put your enhancements back into the new slots.
Plan it first
Write your build down before you start. It goes much faster.
Still stuck?
Post your build in Discord. Someone will help you sort it out.