wanting to go platnium

I'm no expert, but what I would advise (going by personal experience alone) is either starting with your hair as greasy as you can possibly stand to protect it during the lightening and colouring porcesses and toning your hair with a blue-or-purple-based shampoo if it *is* brassy*.

Best thing to do before anything else would be to do a test strand on your current hair, though, and see how it goes - as you're blonde anyway, it could turn out more vivid than you think, hopefully!

covering your hair with coconut oil a few hours before lightening will help reduce the damage that the bleach causes.
If you are in a hurry, one full bleach with 30vol peroxide should be enough to get you light enough, then use a purple toner ( you can make your own by diluting purple dye in conditioner).
If you are not in a hurry, I would suggest taking the longer but less damaging bleach bath path. There is a tutorial topic you can have a look at. A bleach bath will not lighten as much as a full bleach but they are much less damaging and can be done one a week until you get the desired pale yellow and can tone to platinum.
Please make sure to check the color as it is processing as you should never bleach all the way to white. Once the hair is the same color as the inside of a banana, then you can tone it to platinum.
hair which has warm tones in it will always go through a brassy stage, but since you are a natural blonde, you shouldnt have much problem with that.