You need to follow federal law and establish a non-profit charitable organization in order to legally accept donations as a charity - online or by other methods - and have those donations be legal to accept (and possibly tax-deductible to the donor). According to federal law,
Public charities must be organized and operated exclusively for exempt
purposes set forth in section 501(c)(3) of the Internal Revenue Code.
See Starting a nonprofit - GrantSpace, a project of The Foundation Center.
There are considerable stipulations to running a 501; see Exemption Requirements Section 501(c)(3) Organizations | Internal Revenue Service
Depending on the exact goals and type of charity, you may find you need a (c)(4) or a (c)(6) organization, as there are limits on what each category of charity can do with funds, the amounts of funds, and in some cases, if they are even tax-deductible to the donor.
See The Differences Between a 501(c)(3), 501(c)(4), Other Tax Exemptions to determine what you want to do, and then check with the IRS.
If the charity will be based in Florida, check Instructions for Articles of Incorporation (FL Non-Profit) - Division of Corporations - Florida Department of State for special laws regarding charities.
A federally-established 501 in one state can accept charitable donations from any other state, though there may be limits on what those other state residents can give set by their own state laws.
There is no difference in accepting on-line donations and donations by phone, check or mail in terms of what state the donations are coming from. But if you are accepting donations online, you will be required by state and federal banking laws to conduct financial transactions using secure online methods. Those laws and technical requirements are not trivial to adhere to and implement, and the legal penalties and liabilities are considerable.
