Among other things, you will need a contract with the labels providing you content, and you will also need contracts with the various digital distributors (i-Tunes, etc.). (In the case of the digital distributors, you will be using their contracts (which, in many instances, can be revised through negotiations).
And assuming that you are planning to use the model employed by the various existing aggregators (the Orchard etc), you will need the infrastructure to support your operations, such as an online "dashboard." That in turn will require that you have the substantial financial wherewithal to create and operate an aggregator business.
There are a number of other things you will need done by an attorney to make it a fully functional and legally protected business, but the contracts mentioned above would be among the essential starting points involved.
Operating an aggregator company is not a high profit margin business, since you are operating on essentially an override percentage, so you need significant volume to make it a financially appealing proposition.
The above is not intended as legal advice and does not constitute the creation of an attorney-client relationship, as this forum does not provide for a confidential communication. Furthermore, the attorney's answer above is intended to be general information only, and there may be facts not contained in the question which could change the answer, so the answer above should not be relied upon without first obtaining legal advice from your own attorney.
Your question involves two parts.
1. Setting up the legal structure of your business i.e. will it be an Sole Proprietorship, Partnership, S Corp, LLC, or C Corp? Under this umbrella, you will need to also secure a business license or permit in the city or county in which you will be doing business.
2. The second part is the actual licensing agreements you will enter into with various third parties whose music content you seek to aggregate. These involves various types of agreement which is contingent on your facts and what exactly you want in your dealings with these third parties.
In short, your question is quite broad. Lawyers work best when they have the facts which then raises the legal issues that allows them to easily and directly address the myriad of problems that may be lurking in any given business transaction. Your question here is too broad to do much with. You should, at a minimum, sit with an entertainment lawyer for a consultation who can assist you.
Best wishes with your matter.
Ms. Uduak Oduok is an attorney with EBITU LAW GROUP, P.C. The information she provides here is general information. Further, it is strictly based on California law. NOTHING HEREIN FORMS AN ATTORNEY-CLIENT RELATIONSHIP NOR SHOULD BE CONSTRUED AS LEGAL ADVICE. For a legal consultation on your matter, you would need to contact her office directly at www.ebitulawgrp.com.
I note from http://buzzsonic.com/how-to-get-your-music-distributed-on-itunes-and-keep-90-of-the-money:
"A newer breed of distributor has flourished in the current music industry climate, a digital music aggregator, where the artist or label submits/uploads the content and the aggr[e]gator queues it up for placement with the main online retailers, which in mainstream terms means iTunes, Napster, Rhapsody, eMusic (for DRM free indie music) and more recently AmazonMP3. So. There’s a number of aggregators around now and they seem to be multiplying weekly so its important, nay, essential to choose a company with a nice ‘shiny’ reputation. That means a company that is reachable, reputable and accountable and of course, a company with some solid music industry background. I use Tunecore for digital distribution, though you can see a useful comparison of services here via Moses Avalon."
You have your work cut out for you.
The music bsuiness is one of the most complex areas of law in the entertainment industry. During the dotcom boom of the 1990s I actually founded one of the earliest digital music aggregation companies and I can tell you that you'd better have all of your legal bases covered. There is a reason why all major record labels have in-house business and legal affairs departments. You should consult with an entertainment attorney to counsel you. Do a search on Avvo.com