On this page of the guide to Diablo 2 Resurrected you will learn how to start and finish the task Cemetery Of The Sisters - this is the second feature mission in Act 1. In this quest, you will face the first boss - the Blood Raven- and you'll get access to Mercenaries.
Talk to Kashya in the camp. During the Sisters' Burial Grounds quest, you have to defeat the Blood Raven - the first main mini-boss. You will need to reach the Burial Grounds, which do not border the Blood Moor. Go to the Cold Plains - you will know that you are heading in the right direction once you encounter Flavie along the way - she is guarding the entrance to said location. You can reach the Burial Grounds straight from the Cold Plains, however, the entrance to the Stony Field can also be found here.
You can find the Blood Raven in the center of the Burial Grounds - or rather, she will find you. She moves at a rapid pace, uses a bow and has the ability to imbue arrows with elements during certain attacks. As long as you do not have a specific specialization, it's worth using any ranged weapon or magic. In the meantime, you will be attacked by skeletons and zombies (but once you defeat the Blood Raven, most of them will be destroyed). The zombies will surround the Blood Raven to protect her, so you need to eliminate her as soon as possible.
You can also visit the optional Mausoleum in this location. As a reward for completing the task, Kashya will grant you the option to hire mercenaries and she'll even offer you one free of charge.