During gameplay hit the "`" key next to the 1 key on your keyboard. this brings up the console screen. Type "player-> TGM" and hit enter. God mode is now enabled.

Other character modifications can be made in this console. For example to add the complete Daedric Armor to your inventory, key in the following hitting enter after each entry:

player->AddItem "daedric_Shield" 1
player->AddItem "daedric_cuirass" 1
player->AddItem "daedric_greaves" 1
player->AddItem "daedric_pauldron_left" 1
player->AddItem "daedric_pauldron_right" 1
player->AddItem "daedric_gauntlet_left" 1
player->AddItem "daedric_gauntlet_right" 1
player->AddItem "daedric_boots" 1
player->AddItem "daedric_god_helm" 1
player->AddItem "daedric dai-katana" 1

Basically any item can be added to your inventory in this manner. Check the Construction Set for the correct names of items.