A standard OMERO user just needs to download the client package with a compatible version for their institutional server (e.g. 5.5.8 clients will connect to 5.6.0 servers but not to 5.4.0 servers). Full instructions for installation are on the help site.
If you do not have an institutional server, you can take advantage of the OMERO platform hosted by the OME team in Dundee and sign up for a free demo account at https://demo.openmicroscopy.org/signup/. You will received an email with your username and password allowing you to connect to the server in Scotland.