TurboTax Self-Employed Online would be a good choice.
You would want to get yourself set up as a self-employed probably as sole-proprietor to start and since you don't have any employees you can use your social security number, or you could apply for an EIN if you so desire, but don't use the EIN of the employer that was paying your W-2 wages.
Since you have a good idea of what you'll be making then estimating the SS and Medicare by doubling it would be fine. You will actually pay a little less, because you can deduct half of the deductible part or SE or one quarter of the entire amount from your total income on Form 1040. You can also estimate your quarterly tax payments.
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