Continuing...
So, that's the first eight lines.
Notice that the tone now shifts. This is common in sonnets, the first eight lines go together, then there's a shift, then the next six lines say something different.
"But thy eternal summer shall not fade,"--The summer doesn't last long, but your summer will last forever.
"Nor lose possession of that fair thou ow'st."--You'll keep your good looks forever (!)
"Nor shall death brag thou wander'st in his shade."--You won't die (!!!!)
"When in eternal lines to time thou grow'st."--I don't understand this. "Lines" means poetry, this poem in particular, but I don't understand the phrase "to time thou grow'st."
"So long as men can breathe or eyes can see,
So long lives this, and this gives life to thee."--You'll live forever, because this sonnet I wrote to you will live forever.
Now, this is just me speaking... I wasn't an English major, I'm not a poetry expert, I'm just explaining the words. The following comments are only there to help you understand what the poem seems to be saying.
My personal reaction is: what a bunch of malarkey (nonsense)! In the first place, it takes a lot of nerve to say "my poem will live forever." It has in fact lasted four centuries, though.
Second, if someone said "my poem about you will be remembered forever," I'd say, OK, that's nice, but it's not the same as never dying!
Third, it's supposed to be a love poem, but "it's all about him!" Furthermore, since the point is that people will be reading his sonnet forever, it's not really a man writing to his beloved, it's an author writing to his audience of future readers! And, finally, "it's all about him," we learn almost nothing about his beloved or what he thinks of his beloved, except that his beloved is nicer than an English summer's day.
Let's put it this way: "you're nicer than a summer's day" could be written about anyone.