Creating a secondary index will trigger the build of that index automatically.
However, that built is done asynchronously and can take some time if you have lots of existing data to index. To know when that building is done, you can check the nodes log for an entry looking like "Index build of <index_name> complete" (you can also check the IndexInfo system table (SELECT * FROM system."IndexInfo", the quotes matter for backward compatibility reason), a row for the new index will appear in that table when it's built).
You can also force a rebuild however if you want using nodetool rebuild_index.