A relationship (such as a one-to-many) between tables is predicated on the idea that there is more than one table. You only have one table.
For now I suggest setting up a seperate table for employees in addition to one for leave occurences. Also make sure your tables have primary keys (unrelated to any data). Here is a good time to peer into Access's help section and read what is available on primary and foreign keys.
No, Access isn't easy. It is a very complex application development platform. I'm not trying to be discouraging, but be prepared to invest either a good deal of time working up the learning curve, or a good deal of money paying someone else who is already there.