I'm Miles from the Social Care team at Shopify. I'm here to help you out today with getting you set up for a new store.
In regards to your question around deleting the .myshopify.com URL, at this stage, this acts an account number, so it cannot be altered or deleted. If you do wish to start a new store, you'll need to create a new account and a new .myshopify.com URL. For example, if your current store is yourstore.myshopify.com, then you could name your new one yourstore2.myshopify.com. Once your store is created, you can then head to Settings > General and change the store name itself to your actual business name. We'd also recommend purchasing a custom domain (for example yourstore.com) which would mask the .myshopify.com one so customers wouldn't see it. You can learn more about buying a domain here: https://bit.ly/2EasnK9.
Shopify's free trial is designed to allow merchants to work on the platform and make sure Shopify is right for their business. Its intention is not to have a store fully built within the 14 days, but rather get a feel for the platform. You can learn more about the trial right here: https://bit.ly/3hzWDfC.
If you need more time to get everything ready before you launch your store, I'd recommend checking out our Pause and Build plan. This plan is $9 USD/month and provides you with the same level of access our Basic plan - but the checkout is disabled during this time. When you're ready to selling, you can simply switch across to our Basic plan for $29 USD/month which will enable the checkout again so customers can start buying your products. The Pause and Build plan is great for merchants who want more time to work on their store or merchants who would like to take a break from selling.
Whilst you're building your store, have a look at Shopify Compass (https://bit.ly/2LyfedE). Shopify Compass provides you with free webinars and courses on things like store design, marketing and lots more! We're always adding fresh new content to Shopify Compass, so it's recommended for you all merchants from beginners to experienced.
I wish you all the best on your journey with Shopify. Please reach out if you're ever stuck. We have 24/7 live support all year round, so you're never alone.