Welcome to the site, clindahl84 - Nice to have you with us :)
I agree with fudjj, its really hard to make an equation out of it. You have to give space for the unforetold's/unforeseen's and if you don't achieve your goal you'll end up getting frustrated.
But it's good that you have a set target to aim for, it gives you cause to work hard. Just don't be too hard on your self though.
And remember to have fun while you're at it! Fun will make everything a whole lot easier to achieve. So if you can start with products are you are interested in.
I'm not sure where you plan to sell online but in case you will go with eBay, we have a number of very useful guides that you can start reading while deciding on your product line...
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Hope this helps.