Welcome. The POD starts when the Byzantines make a truce with the Turkish invaders right after a hard-won Byzantine victory. This meant that the Bosporus Strait was still functional as a trade route between the European nations and the Eastern states.
The new continent before its colonization.
Stories of a land to the west of the Atlantic were common among the Vikings, but must of them were considered myths and folk tales by scholars during the Middle Ages. This land was populated by different tribes and people, the Tawantinsuyu, the Aztecs, and the Mayans, being the most successful native states.
Although relatively isolated from the rest of the world, trading between tribes and cultures existed. After the Inca Civil War, the Tawantinsuyu expanded to the area that is OTL Central America, making contact with the Maya and Nahua people, starting a thriving commercial relation between the Inca Empire and the other smaller states. Conflict within the area was common, specially with the Tenocha State, the last Altepetl of the old Aztec Empire.
The first contact with the Europeans.
As noted before, Scandinavian Vikings were the first Europeans to arrive to Alexandra. Many of them would raid the natives territories, while other were more interesting in trading. Leaded by Erik the Red, the first place they reached was OTL Greenland, where they settled in the southern part of the island. After this, Norse explorers went south and reached OTL Terranova. The land was abandoned by the Norse, but stories remained of the land of Vinland.
During the Christianization of Scandinavia, many Norsemen escaped their homeland to avoid persecution by the Catholic church, many of them reaching Greenland, while others escaped to the mainland in search of Vinland.
Thanks to this Norse migration, the Natives gained new trading opportunities, but were also exposed to the different diseases brought by them.
Colonization by the European powers.
In 1741, Russian explorers reached Alaska, but it was in 1742 when Alaska was finally settled by the Russian Empire. The continent was named by the Russians in honor of Alexander I of Russia, who was crowned in 1801.
After the Russian expedition, other European Nations wanted a piece too, and the Age of Imperialism started.