My advice? Forget "proof" and instead focus on "reasonable doubt". Also, don't focus exclusively on Egyptian mythology. Instead look for similarities to all existing mythologies from the time period in which early christianity was getting it's feet beneath it.
See if you can find The Power of Myth
and/or The Hero with a Thousand Faces
by Joseph Campbell. Also, check out Lord Raglan's Scale
. Lastly, check out The God Who Wasn't There
(if you can find it).
One last word of advice: 99.999% of those that will tell you that there are no similarities are going to be christians. This means that they have a bias and/or an agenda. Consider their arguments with this in mind. Do your own research. Take a look at what you find with an open mind and healthy sense of skepticism. And keep in mind what I said about "proof" vs "reasonable doubt"