The answer to this question really depends on what time period you are asking about. Generally speaking though, there are two ways to trade between Italy and Northern Europe: by sea or by land.
Starting in the Bronze Age, limited trade around Europe existed. The Mediterranean region, including Italy, was well connected with trade routes, and goods flowed throughout this region. Some Mediterranean goods such as minerals and spices did make their way to Northern Europe, but the exact routes are not known. As bronze smelting became more widespread, tin from Northern Europe grew in demand in all regions, including Italy. It is likely that trade routes went overland, given the limitations of sea travel at the time. The actual trade routes are unclear, but they likely involved many short legs and passing goods through the hands of many middlemen.
By this period the Amber Road was well established between the Baltic Sea, through what is now Poland, and the Mediterranean. This route, which transported a lot of the precious material amber, thrived all the way through the Medieval Period.
By the Roman period, a single empire connected Italy and Northern Europe. This increased trade, as Rome was fond of goods from all over the empire. The Romans built roads and maintained mountain passes through the Alps. This allowed goods to be traded by land much easier than before. As shipbuilding technology improved, more trading by sea took place. The most common route was to sail around the Iberian Peninsula and out into the Atlantic to reach ports in Northern Europe. This route was quicker than going overland but ran the risk of encountering dangerous storms and currents. The Romans even used rivers to trade. Ships could sail from Italy into the Black Sea, then up the Danube into the interior of Europe. This was a longer route but proved to be much safer. Given the indirectness of trade by ship and the difficulty of mountainous land routes, trading was risky and time-consuming—and therefore expensive. At times the Roman government subsidized merchants trading important or high-demand goods.
In the Middle Ages and Renaissance, Italian merchants were at the center of trade. Many of the old routes from the Roman Empire continued to be used. However, with different kingdoms and city-states along these routes, trade became a little more complicated. Taxes were often charged when passing through different territories by land or stopping in foreign ports. This increased prices along the way. By the Renaissance, Europe was better connected through trade than it had been since the fall of the Roman Empire. Merchants from Northern Italy were becoming some of the wealthiest people on the continent. While trade was still risky and expensive, these merchants were able to make great fortunes by trading valuable goods, particularly silks and spices from Asia, all over Europe.