I am planning a trip to Tokyo but I don't know anything about customs. I will be leaving Salt Lake City with a layover in California. During my layover do I have to go through customs before I board the plane going to Japan? Also if i fly Delta to California and then take a Japanese airline where do I get my boarding passes for the Japanese airline?
There's no customs inspection when travelling through or leaving the United States.
You will have a customs inspection after passport control at the first airport you come to in Japan. It is nothing too much to worry about, your bag might be opened in front of you and someone might look through it for any illegal articles. Sometimes you are allowed to go through without a detailed bag examination, usually only if you are carrying a small bag.
Where you get your onward boarding passes from depends on how you booked the travel. If you booked it as one ticket all in one go, you'll probably get all your boarding passes for the day in Salt Lake City. (I am not aware that Delta cooperates with either of the Japanese airlines so I am surprised that this would be one ticket.) Otherwise you can collect your boarding pass in LAX, from an airline assistance desk. If you are taking a bag you may have to exit the airside area to collect it and re-check it, in the check in area. You can get your boarding pass there as well.
You're basically going to have to go through customs upon entry in each location. If you're flying to Tokyo direct, that means you go through Japanese customs upon arrival. Keep in mind that if you have a layover in say, Vancouver, you'll have to go through customs both in Canada and Japan.
Boarding pass for the Japanese airline should be made available to you online by whoever you booked the reservation through. Worst case scenario you can pick up your boarding pass at the reservations desk in California. Only downside is you may have to exit and re-enter security to do it.