First off, there is no "easy" way to start a business. Ultimately that's what you're doing you're offering a service and starting a business. You're doing more then just "coaching" If you want to run a program that already exist try Middle School Cheer or Youth League.
Second, Yes I would recommend assistant coaching a youth league team that is already established to get some experience in. However, All stars is different from youth league and if you truly want to do all stars I would suggest writing your local gyms about volunteering with a mini or junior team to get some experience under your belt. Coaching isn't easy, sometimes you will have to change up things on the spot, you will have to replace and train fliers, bases, backspots etc. You have to keep parents informed about a lot of things if not, they will ride you with questions and if you don't have an answer for them you might not get a good response. You have to organized as well.
It is also important to get certified as well and CPR and First aide certified. It makes you look creditable as a coach and it shows that you care about the safety of your students. If you do go the youth league way they're many online certifications that they have such as AYC, Pop Warner (which is now USASF I believe) and so on. I would just recommend going ahead and getting USASF certified because it last a few years and it really gets into detail about a lot of things you think you may know but really don't. Plus its easier to get into USASF Sanctioned competitions being USASF certified. Its only very few Non sanctioned competitions that uphold good quality events that doesn't require you to be certified.
You also need to take business courses you can do this at your community college. "intro to business' is a good place to start I have to take that class in the summer to get my administration degree. You wanna take that class because it just teaches you 101 things about starting a business and being an entrepreneur. You also have to decide what type of business you want to be. Do you want to be non profit? how will you qualify for it? non profit 501 c3 doesn't work for everyone. Will be LLC? (Limited Liability company) and the list goes on. What type of insurance are you going to have an how many? Are you just going to have Accident and liability, are going to have insurances for students who may experience rape/molestation in the gym(It does happen) Are you coaching all the teams? Are you going to hire help? will the background check? whos' paying for background checks? are you? are your employees? will you reimburse them? Where will you get mats from? where you will find a building that is safe? how many comps you will attend? How will you pay your employees? what's your business plan? I can go on forever and ever. It is really important to know this. If you have to ask "what's the easiest way" to start a business you're not ready to start a business. There is no easy way to start a good quality, safe functioning business.