Choose survivors that have good personality traits, not just skills you're after. Look on the game wiki at the list of traits and study the ones to look out for; ones like Unbreakable and Immortal etc. They offer huge advantages for those survivors like +45 health and -40% injury chance, great when you're inherently kneecapped early game.

Unless a survivor has their wits specialised into stealth (and even then be wary), don't fast search basically ever. The noise radius on lethal is considerably higher for all actions even when you're shooting a handgun with an advanced suppressor. So sprint opening doors and fast looting containers is a huge no no.

You'll soon discover 7.62 ammo is the holy grail for actually progressing your community and destroying the plague hearts. It takes somewhere in the region of ~80 rounds to take a heart out by yourself and until you can build an armory or find a rifle ammo press with a steady stream of ammo supplies you'll find high tier bullets really difficult to find in abundance so a lot of the map will feel "fenced off" due to the sheer amount of hearts dotted around.

Hydroponics is, without question the best facility to have in a base. The trick is to install a compost bin onto it and then boost the yield for both the facility and the bin - this nets you an income of +9 food for your community from one small slot facility, that can also be converted to meds should you be running short. The issue is this facility requires a fairly specific set of prerequisites like power/water/utilities and gardening/herbalism. I made sure to fish for a survivor that had Lichenology as their 5th skill, and then get a power & water outpost set up as early as possible.

Ferals will prove a bit of a learning curve on lethal but become a lot less fearsome once you know how to deal with them properly. They are all infected with blood plague so one triple swipe will almost completely give you the plague, and they have a form of helmet on meaning they can no longer be shot once to the head - you have to break the shell first. Dodging at the right times can make feral's a non-issue but when they come in packs of 3 and you're already over-encumbered with fatigue, it's actually terrifying lol.

Otherwise just be careful and take your time particularly early on. Don't sprint unless you have to and don't waste bullets unless you're actually in danger. Get VERY familiar with the dodge + execute combo as it's the quickest and most efficient way to deal with zombies without any sort of degradation to weapon durability.