You tap the back button (probably says Mailboxes) above the mail list until you can see your Inboxes and Accounts. Choose the account and navigate to the folder. When you want to go back to the Inboxes just tap back as before.
If you only have one mail account, it may be easier to just select the Inbox folder under the Accounts area. That way you'll be able to get at the other folders more quickly.
I'm so glad Twerppoet was here to instruct you so well!
The IPad Forum is a community of members from all walks of life and from all over the world coming together to share our experiences and to help each other learn about this wonderful piece of technology the Ipad. There is a wealth of information already here in the many threads we have so doing some searches will answer most of the questions you have. If then you cannot find the answer you are seeking then by all means post your question. There are many friendly and informed members here only too willing to help you.