Since you are new there are a couple of topics you'll want to spend some time reading up on. It's not difficult, but understanding the fundamentals will help immensely.
First, check out table calculations. You can get the % of Total (or running sum, or running average, or running %, or difference from, etc...). There's a lot of customization possible. For example, I attached a worksheet to demonstrate, but I wasn't sure if you wanted the % of Total for everything or % within a category -- no matter, just customize the table calc and your good to go.
Second, I used Measure Names / Measure Values to show Sales and % of Total Sales side by side. This can be a little confusing at first, but just remember that these fields can be used whenever you want two or more measures to share the same space on a visualization (same axis, same pane, same color shelf, etc...).
Hope this helps!
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Here are some resources:
Table Calculations: http://www.tableausoftware.com/table-calculations
Measure Names / Measure Values: http://kb.tableausoftware.com/articles/knowledgebase/measure-names-and-measure-values-explained