Tap Titans run-down, as I've been playing it a lot:
1. There are good heroes, and there are bad heroes. Only upgrade good heroes initially. Good heroes are heroes with +0.4% tap damage, gold drops, and +% hero damage twice. Mila, Macelord, and Skulptor are DPS machines early. Upgrade your older heroes once you have a ton of money to blow. Usually I wait until the -gg stages of money then go back and level all the older heroes to 800.
2. Salvage shitty relics. Don't be afraid to salvage them. Go for ones with +gold, -Midas cooldown, and +Midas duration, if you can. Relics get really expensive, so salvage the bad ones. It's worth it.
3. Tap damage is amazing early, pretty awful in the 60-200 stage level range, then it becomes your number one source of damage 200+, especially with the crit relic. Do not level your main character past level 600, it's completely pointless. Get the +0.4% damage increase from your heroes when you can, it's the best upgrade bar-none.
4. Don't push yourself through a boss. Every 5 levels you face a boss that can kill your heroes. Go all the way up to them, and if you can't kill them in 10 seconds, don't risk it. Just tap the "leave" button in the upper right. There's no shame in farming for a bit then going back, as there's no worse feeling than having 90% of your DPS die from a boss, only to leave you worthless for 10+ hours.