How does this practice shift the equilibriums (price and output) for tobacco and domestic food items (analyze both the local and international effects)?

This is the easy part of your question.

Draw initial world-market supply and demand curves for tobacco; and another for domestic food items. If the US subsidises foreign tobacco farmers, foreign tobacco farmers produce more, causing the price for tobacco to fall, causing US producers to produce less. But, overall, an net increase in world production. If the demand for tobacco is inelastic (likely) then the overall amount of money spent on tobacco will fall. So, we should see a world wide increase in demand for other items.

The hard question to answer is why the US would subside foreign tobacco producers. Who knows. Perhaps the US is committed to raising incomes of third world countries. Perhaps the the US would rather the farmers produce tobacco over something like opium-producing plants. Perhaps the US trade officials are corrupt.

I hope this helps