There is a web.config setting you could use
<add key="PageToRedirectToAfterSignIn" value=""/>
However, my recommendation is to not override the existing logic with this setting. The existing logic is that whatever page the user is on when he clicks the login link, that is where he will be returned. This is helpful for forums when a user wants to reply or post but needs to login first he is redirect back to the correct forum page. It is also essential for WebStore if a user adds an item to the cart and wants to checkout but need to login first he will be redirected back to the correct page in WebStore. So if you override this to force a redirect to a specific url you lose a lot of value for the user and perhaps even lose money if a user does not follow through with an order because he logged in and was strangely redirected away from the store.
Hope it helps,