Your question is a bit vague. Hopefully I will give you enough to get you moving in the right direction. Create your string collection (which is like an array - but don't set any data constraints on them). Then you can add items to this list in your work item. For example, if you are using 3.0 or 4.0 pre 3, you could create an edit box and a button. When you click on the button, simply use an update variables step to add the value of the edit box to your string collection.
To display the list, you could create another edit box, set it to allow multiple lines. In the page initialization action, create a loop that loops through all the items in the array and appends them to a string with "\r\n" (carriage return / line feed). If you are using 4.0 SP3 or later, simply use a datagrid to display the list.
If parts of that are unclear, let me know where you're stuck and we'll help out.
--Jason