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:
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: