The tasks you create for a usability study are essential for gathering the right data. The tasks determine what you'll test and impacts what parts of the design your team fixes. If you give users the wrong tasks, you risk focusing on the wrong parts of the design
Set tasks that are essential to the new site's success, such as:
Buying products
Paying bills
Contacting the client
The site was built for a reason - can your target audience do what you need them to do?
Think about all of the different ways your product or web site will increase revenue or reduce organizational costs. The best tasks focus on areas of the design crucial to your organization's business goals.
If there are certain areas of the design where you have little knowledge as to how users interact with it, this is another area to address in your tasks.
It's also a good idea to ask the user to suggest tasks. While this gives another indication of their expectations and requirements, it may suggest new functionality or priorities
How many users do i need ?
Effective User Testing Doesn't Have To Be Extensive
Jakob Nielsen's study on the ideal number of test subjects in usability tests found that tests with just five users would reveal about 85% of all problems with your website, whereas 15 users would find pretty much all problems.
Write usability test materials in clear and simple language
This will be especially important for participants with some types of cognitive disabilities who have difficulty processing information or instructions.
Existing customer - My account 
Optimize your shopping cart.
Navigation Related Usability Tips
Preparing the test
Optimizing E-commerce user experience
TOP 10 tips to improve your website Usability