Default help with google voice

i have couple questions about google voice .I have looked every place
1) does google voice run off the data or wifi on my phone
2) when i uses my phone to call someone it asks if i want to use google voice or phone to call out when i use google voice am i still getting charged for my cell data
3) does anyone have a good how-to install google voice so i can use it on wifi seams lot of post on it are dead and stuff has changed i used and i can't seam to get anything to work with them