I'm been having issues w/ my truck? There is about 150,000 some miles on it an it feels like it's struggling to drive or accelerate? Auto Zone said O2 sensor and/or 6th spark plug fouling? I had a friend whom is a bit of a mechanic an said he thinks it's a head gasket. What steps should i do to find out, an i really don't want to dump a bunch of money into this truck if it's going to be a huge expense to fix it...

 

02 sensor sounds like a more likely idea. If it were a head gasket you'd have more of a problem than sluggish acceleration. blown head gaskets will have tell tale signs like coolant in oil, oil in coolant, and over heating.