How would I go about adding a trial version and achievements for my Windows Phone 7 game?
And how can I make the game upon finding it's a trial prompt the user to purchase the game and redirect him/her to the games page on the AppHub?
For a trial version you can use the IsTrial API call - information about it on MSDN - you call this once in your game and assign a TrialMode value in your game to it as checking this in a Game Loop is not recommended, then you can have logic in your game change depending on what this value is - the Marketplace process takes care of the process of Try & Buy once you mark your app during the submission process as having a Trial mode.
Achievements are only available for Xbox LIVE enabled games which requires a special publishing relationship with Microsoft, I'm not sure of the process but try the create.msdn.com App Hub website, there may be information there, this is not open for all developers, as even now there are very few Xbox LIVE enabled titles in Marketplace