First you need to subscribe for plan according the credits you need.

You need to find your own customers. Here are some tips:

You can create a website
Send post on online forums
Create a social media channel like Instagram
Email Marketing
You also can find customers locally.
You can price service for your customers as you want. For example you can send a 1 month line 12$ to your customers.

Whenever you sold a subscription to your customer, open reseller panel, create a line and send it to your customer.

You can ask for more credits whenever you need.