How do I get Torrent?

Accept Melina's accord to receive the Spectral Steed Whistle, then rest at the Gatefront Ruins or Agheel Lake North grace.

Where do I fight Margit?

At the Stormveil gates in Limgrave. You meet him again in the Capital Outskirts.

What is Margit weak to?

The wiki lists a weakness to slash damage and vulnerability to bleed.

Elden Ring Early Game: Get Torrent and Beat Margit

Margit, the Fell Omen is an early boss that stops many new players. The fight looks scary, but the facts show several ways to make it easier. Everything below comes from the Elden Ring Wiki.

Key takeaways

First fight: Stormveil gates in Limgrave. HP 4,174, 12,000 runes.
Second fight: Capital Outskirts. HP 8,193, 16,800 runes.
Weakness: a 10 percent weakness to slash, and vulnerable to bleed.
Phase two starts at 60 percent health.

ELDEN RING Shadow of the Erdtree – Gameplay Reveal · YouTube

Part 1: get Torrent

Meet Melina and accept her accord. She gives you the Spectral Steed Whistle.
Rest at the Gatefront Ruins or the Agheel Lake North Site of Grace, or touch three graces and rest at another.
Use the whistle to call Torrent in open areas.

Heal him with Rowa, Sweet or Frozen Raisins. Crimson Tears work only while you ride. If he dies, the whistle revives him for one flask charge. He can double jump, but you cannot swap one- and two-handed grips while mounted, he cannot enter some indoor areas, and a fall from too high can kill you.

Part 2: beat Margit, the Fell Omen

	Stormveil	Capital Outskirts
HP	4,174	8,193
Runes	12,000	16,800

Tools that help

Margit's Shackle: stuns him up to twice in phase one for five seconds. It does not work in phase two.
Sorcerer Rogier: can be summoned for help.
Bleed: Margit is vulnerable to it.
Slash weapons: deal extra damage, because of his 10 percent weakness.