Drafting a Contract
If you're selling a service or signing up your first few users, you'll want to setup a contract to establish mutual terms of business.
The easiest way to do this is by using avodocs (opens in a new tab).
It has document templates in place that you can use to create contracts in seconds.
Signing documents
There are many options to send contracts to your customers to sign.
I personally use the free tier of xodo sign (opens in a new tab).
Note: Always download a copy of the signed document and save the copy on google drive or anywhere safe.