I would recommend the Source engine, its free well balanced but its in C++.. I am not sure why you’re learning C# but I feel it’s sort of a middle langue, its powerful and limiting but similar to its bigger brother. The Source SDK can be download free thru Steam if you own a Source game(TF2, Half-Life 2, Day of Defeat: Source, Counterstrike: Source, Left 4 dead, Left 4 dead 2). It also has a great community to help you out with anything you need..
http://developer.valvesoftware.com/wiki/Main_Page
I would also check out www.interlopers.net/
Included in the SDK is the full code for a game from scratch.. Or a game built/starting with the Half-Life 2 Deathmatch.. Which is basically HL2 with out the SP maps.. All weapons and object are there so its good base to start coding small things here and there as you learn.. If you with to continue its going to be hard but remember to comment your code it will save you later