truck is a 2012 ford f150 ecoboost 3.5L v6 I just bought a boost guage for it where do I hook it up to the vacume line? idk where that even is can you help me at all

 

Remove the plastic engine cover and near the rear of the passenger side valve cover there is a vacuum line that you tap into. You may need a T-fitting.
Below is not my pic, on my 2011 I routed the boost line so it ran back towards the firewall. On my 2014 the vacuum lines were different so I had to tap in near the throttle body. But this pic should look the same as your 2012.