I was always taught to capitalize the first letter of the first word in a sentence, and also the first letter of proper nouns. In the last few years it's been common for certain firms to name their brands something that is always spelled with a small first letter and a capital second letter. It is almost as if they demand the rules of usage are changed. What do you do about this? Where is the inquisition when you need it? Should you start a sentence with "IPhone"? Should you use "iphone" in the middle of a sentence?
You should never change a brand name. 'iPhone' should always be spelled as 'iPhone,' no matter where in the sentence it is. 'IPhone,' 'iphone,' 'I-phone,' 'i-phone' or 'I phone' are always wrong. 'iPhone' is the only good one:
iPhones are the best selling smartphones.
IPhones are the best selling smartphones.
Iphones are the best selling smartphones.
This is the same for all brand names, but this can also be for other (nick)names invented by people, for example 'rms' which should always be spelled lowercase.
There could be, however, one exception, when the sentence is spelled in all caps, usually for styling purposes:
IPHONES ARE THE BEST SELLING SMARTPHONES.
iPhones ARE THE BEST SELLING SMARTPHONES.
Wikipedia suggest that eBay is the correct usage.
eBay announced that starting in March 2008, eBay had added to...
However, I would be interested to see if the same convention applies with abbreviations like mRNA
Again, Wikipedia suggests the same.
mRNA is transcribed from a DNA template, and carries coding information...
However, that's not to say that these are the "official" rules in any capacity.
iPhone is tricky. You have to capitalize it the way Apple does. It is a product therefore has to be labeled the way the brand does. Technically anyway you write it would be correct but if you want to be exact then capitalize the "P" only even it is a the head of a sentence.