How do I set the isolation level for SELECT statement?

Using PostgreSQL 9.5 I would like to make sure that when I run a SELECT statement I get consistent results, i.e. if the table is modified while the SELECT is running, I want to get the state that was valid when the SELECT started. Basically, it's like: Take a snapshot of a table, and don't allow updates to the table while the snapshot is being taken, and then deliver the snapshot, while updates are allowed again. If I understood isolation levels correctly, REPEATABLE READ is what I'm looking for, isn't it? Now my question is: How can I run a SELECT with REPEATABLE READ isolation level? Or am I missing something, and my approach is wrong? How would I solve this?