This is a very big question. Reason being, there were probably hundreds of effects of westward expansion. But, there are a few that come to mind.
1. Indian Wars
2. Economic Boom
3. Increase in Amounts of New Immigrants
4. New Issues Involving Slavery That Led up to The Civil War
5. Manifest Destiny Achieved
6. United States Increased Trade Opportunities by Being On Two Sides of North America
7. New Infrastructure Jobs
8. Gold Rush
9. Lawless Territories
10. Disease Spread From East to West