You can buy items from the shop by typing !shop in chat while playing on a server. This command will draw a menu with multiple item categories.
Selecting a category will draw a list of items in that category. You can always press 8 (back) to return to the category list.
You can select an item in the item listing menu to view more information on the item such as its description and pricing.
If the purchase button is blocked out for any of the purchase types, it means that you do not have enough Titan credits to grab that deal.