Mate if you've never touched a weight ever you will find growth will probably happen regardless of diet,BUT having a great diet from the start will definately help things along.I'd say that in the start you will be able to drop that gut and add mass at the same time. You weigh 145 so at 1.5 grams of protein per pound of bodyweight thats about 210 grams a day, spread it out over 5-6 meals and eat enough carbs and essential fats to keep energy up.Eat most of your carbs straight after you train along with a good whey shake.Dont just cram calories in at any cost you wont need to yet just add little bits here and there.

The 2 other things to consider are the routine of course and sleep get at least 7-8 hrs a night and you are good to go

Hope this helps somewhat.

Cheers HIThopper