Tower allows creating a new local tag in a couple of ways.
Perform one of the following actions:
In the Sidebar drag and drop a local branch on the "Tags" section header. The new tag will be pointing at the dragged branch's current commit.
In the Sidebar right-click on the "Tags" section header or click on the "plus" button at the bottom and select "Add New Local Tag...".
In the Commits "List" view right-click on a commit item and select "Create Tag from This Commit...".
In the Commits "List" drag and drop a commit on the "Tags" section header in the sidebar. The new tag will be pointing at the dragged commit.
In the Commits "Recent" view or in a commit's details view click on the option gear button and select "Create Tag from This Commit...".
IMPORTANT: In Tower you can only check out branches - you cannot check out tags or individual commits. However you can select a tag and select "Create New Branch from Tag" form its contextual menu or drag the tag onto "Branches" in order to create a new branch based on the tag's commit.