You can write and edit your terms and conditions by going to Settings (the gear-looking icon) and then to E-Commerce -> Order Settings.
On this page near the bottom you'll see a section of test with terms and conditions. You are welcome to use what we've chosen to have in there by default but of course you should read through it and edit it as needed - make it your own, this bit of text is essentially an agreement between you and your customers and they will see it and have to agree to it when they sign up for an account with you. You can also have it show each time they submit an order if you're like.
First, you'll notice that you'll need to add your own company name in the appropriate spots, as it starts out as a generic template. See below:
As you're reading over the T&C's make sure you're adding your own company name anywhere you can see generic references like above.
After you've gotten that taken care of, the last thing to consider is if you want the customer to have to acknowledge/agree to your terms just once upon creating an account as your customer OR if you want them to have to do it each time they submit an order to you!
If you'd like the latter, you'll just use the setting directly above the text box:
