A few tips
Pre-workout
Before you workout you want your body to be prepared. Don't eat anything that will make you feel sluggish during your workout. I usually have a banana or a small shakeology shake about 1.5hrs before. I also use E&E (energy and endurance), to give me a boost for the workout. I have noticed a big difference when I take E&E from when I don't, so I really like to take it.
During the Workout
What if you get super tired and need break during the workout? Take one. Can't do some of the exercises? Modify. You will notice that as you get further into the program you will be able to get through more and more of the workouts without extra breaks. You will also get more coordinated at the moves and be able to do those crazy ones that you laughed about during the first week because they were too hard. I surprised myself many times during my first round of insanity
Post Workout
Some sort of recovery drink after a hard workout is best. I like the P90X Results and Recovery because it makes a big difference in the recovery of your muscles, plus it tastes really good. You can try this, or low-fat chocolate milk, or even just eat something healthy, but make sure it's in that first 0-60minutes post workout, because it'll make a big difference in your recovery.
Nutrition
Nutrition is the biggest challenge for most people. Track what you eat and you'll see the results you are looking for. This is going to be at least 80% of your results. You can do Insanity, eat bad food, and see almost no results. Nutrition is vital to achieve your goals!
Motivation
Some sort of accountability group is going to benefit you the most. This thread is probably going to be what helps you the most through your journey. You aren't alone! You have all of us here to support and motivate you

Andrew