Ballotpedia is a good resource for finding current information on ballot propositions in each state across the country. It is also possible to visit the Secretary of State website of a given state to locate a guide to upcoming ballot measures.
Each state has different mechanisms that are used to present ballot measures to voters in a general election. Some states, like Texas, vote on a variety of amendments to the state constitution, while other states allow citizens to advance a ballot measure through signatures or petitions. Because the laws change from state to state, finding the ballot propositions on a particular Secretary of State website is the most reliable route.