Building a base is the most important part of the beginning of any program. Two of the biggest mistakes made are doing too much early on in the build-up phase and running too fast.
The object of building a base is to slowly build up your aerobic conditioning. If this is done too quickly you will likely end up injured. Most newcomers make the mistake of overdoing it early on and never recover.
At the very beginning all running should be done at conversation pace. If you cannot speak coherently (assuming you could while not running) while running you are going too fast. Slow down and enjoy the run. Typically this would be about 60-75% of your maximum heart rate, but if you don’t have a heart-rate monitor or don’t want to be constantly checking your pulse the easiest thing to do is the talk test. If you cannot talk you are going too fast. The time for speed-work will come later in the program, right now your focus should be to get time on your feet running.
I’m not sure how long your program is and if you are starting from “scratch” but the base building should take about 3 months. You should increase your mileage each week by 10% during the building phase. That can be done by either increasing each of your runs, or if you are only running 3-4 times a week the easiest way to increase is to add an additional day of running. You should discuss your mileage with your coach and be careful to adjust as injury/illness/life changes your plans.
Good luck and be careful out there!