You may start the organization as an LLC - that should be a separate organization and there should not be unrelated business activities.After the organization is formed and there are bylaws in order to get tax exempt status from the IRS you will have to file Form 1023 to apply.
Here is the form
Extensive information is available at the IRS site.
If you want to see an in-depth discussion of exemption requirements under section 501(a) of the Code of an organization's eligibility to apply for recognition of exemption from federal income taxation
IRS Publication 557, Tax Exempt Status For Your Organization - http://www.irs.gov/pub/irs-pdf/p557.pdf
IRS Publication 4220, Applying for 501(c)(3) Tax-Exempt Status - http://www.irs.gov/pub/irs-pdf/p4220.pdf
When no additional information required - processing time takes 6-10 weeks. About half of 501c3 application are completed within that period.
In other cases - the IRS sends an inquiry letter (form 1312) - and that delays the processing especially if applicants do not respond promptly.
In any case - it would be reasonable to expect to complete processing and receive a determination letter within 9 months.
The IRS published Top Ten Reasons for Delays in Processing Exempt Organization
Applications - http://www.irs.gov/Charities-&-Non-Profits
Let me know if you need any help.