The Doom of Valyria occured 5000 years before GoT, when every ruling house in Valyria had dragons and House Targaryen was a minor house that had little prospect. Around the same time the Doom happened, the Andals crossed over to Westeros and began their war with the Children of the Forest, who lived in Westeros with their Weir wood forests. As the children of the forest lost battle after battle to the superior numbers and army of men, in desperation the children created the White Walkers.
A picture of Valyria right before The Doom. Hopefully its more Conan than Wonder Woman. You'll probably see two threads in the new show, Valyria day to day life (think ancient Sumerian, Babylonian, Egyptian civs) before "something" fucks up their entire civilization and another thread of Andals conquering Westeros, aka European White conquistadors invading South America
meanwhile in modern day Westeros