My understanding of けど/だけど is that it primarily means "however." So you sentence is more like "My friend is coming, however I will buy pizza and unhealthy food."
What you probably want is だから or それで, which work like "therefor."
If... if, if, if. There are a lot of ways to say "if" in Japanese, each with their own peculiarities. I think here もし... たら is the best form, which is like saying "If... Then."
Basically, with a verb, you conjugate it into the plain past and slap a ら on the end.
Note that I also changed に to が, as I believe that your sentence was actually saying "I will come at my friend."