You don't need any special equipment. Your phone's built-in microphone is more than good enough, and most recordings come through beautifully.
A few simple things make a real difference though.
Background noise
A little background noise is completely fine; the hum of a car, birds outside, the quiet sounds of a house. These rarely cause problems.
What makes transcription harder is loud, unpredictable noise like a busy café, a room with a TV on, or a crowded space where voices overlap. If you can find somewhere reasonably calm, even just stepping outside or sitting in a parked car, the audio will be much cleaner.
Recording with another person
One-on-one conversations work really well. Two voices, taking turns, are easy to follow and transcribe clearly.
Where it gets tricky is when three or more people are talking together. Even with software that can identify different speakers, conversations that overlap or interrupt each other become difficult to untangle and some of the detail can get lost.
If you're recording a group conversation, the simplest solution is to record a voice note afterwards instead. Capture what was said, what stood out, and the moments you don't want to forget. You'll often find you remember more than you think, and the result is just as rich.
The short version
Somewhere reasonably quiet, one person or two, phone nearby. That's really all it takes.