Quest priority

Focus first on quests that raise your trader level. Higher trader levels unlock better weapons, ammo and gear. Early Prapor and Therapist quests give fast XP and rubles.
Some quests are prerequisites for others. Following the chain prevents unnecessary waiting. If you have several quests on one map, do them together to save time.
The Hideout: passive power

Upgrading your Hideout modules grants passive perks: faster healing, more stash space and even passive income.
Early priorities: Stash (storage), Ventilation, Medstation and Nutrition Unit. The Bitcoin Farm provides passive rubles long-term but is expensive to set up.
The long-term goal: Kappa

Kappa is the large secure container rewarded by 'The Collector' quest, which unlocks after completing almost all quests. It's the most prestigious goal of a wipe.
As a beginner, don't obsess over Kappa yet; but knowing which quests are required for it tells you which ones not to skip.
PRO TIP
Accept a quest before working on it — some require items to be 'found in raid' (FIR), and only those collected while the quest is active count.
WARNING
Never sell FIR quest items early. Re-finding them, or buying from Flea (if possible), costs you both time and money.
Frequently Asked Questions
Which quests should I do first?
Early Prapor and Therapist quests are the ideal start: fast XP, rubles and trader levels. Group quests on the same map.
What is the Kappa container and how do I get it?
It's the reward for 'The Collector,' which unlocks after finishing almost all quests. It's a large secure container and a long-term goal that can take months.