From Shopify Wiki
Setting up your store
There are 2 parts of your store: The storefront and the admin. Everyone will be able to see your storefront, but only you will have access to the admin.
To see your store's Storefront go to the URL http://your-stores-name.myshopify.com and replace your-stores-name in the URL with your store's name. For example, if your store's name is Threadify then enter http://threadify.myshopify.com.
To access your Shopify admin you need to append /admin to the URL. Using the above example your admin would be http://threadify.myshopify.com/admin. After you log into your admin you will be able to manage your store.
If you want your store to accept monetary transactions (credit cards), you need to select a supported payment gateway. Shopify recommends PayPal, but there are many worthy alternatives. Once you have selected a payment gateway go into your Shopify admin and go to the far right tab labeled "Preferences". On the right-hand sidebar click on "Checkout & Payment" and select your payment gateway.
Using your own domain
Shopify gives you a free yourstorename.myshopify.com domain. If you want to use your own custom domain then you will need to contact your domain provider. Learn how to change your domain address
When you log into your store on the far right you will see a tab labeled "Preferences". From the preferences page you can edit your store's settings (including Shipping Options, Domains and Taxes). Learn more about individual preferences.
Customizing your storefront
Most Shopify themes allow you to customize your theme by clicking on Assets in your admin and then selecting Theme Editor in the sidebar. From here you can do things like upload your company's logo or change colors. Learn more about designing Shopify Themes.
The next steps
For a more a more detailed overview on Shopify's features go to Using Shopify.