Why is my truck starting up fine, then cutting right off?

What is your question?

When I got the car it needed a jump, and started right up, after sitting for months. I even drove it home a 1 mile away, no problem. Now I have a new battery and maybe a gallon of gas in the car, but when I start it up it cuts right off after 2 secs or so.....Help me!.....What could it be, is it the battery not having enough charged or does it just need more gas?