Here’s a list of recommended kit to get you started with off camera flash:

Yongnuo Flashes are affordable, powerful and easy to use. This one has manual power control, is reliable and has a great power range:

Then you need some triggers:

Canon version:

Nikon version:

If you have another brand, either of the above will work anyway as a basic trigger.

And an umbrella to soften the light:

Then you need some triggers:

Canon version:

Nikon version:

If you have another brand, either of the above will work anyway as a basic trigger.

And an umbrella to soften the light:

Something to put your flashes on:

And to attach your flash to your light stand, you need this: