You can find the form easily on the Glasgow City Council web site (on iphone and it won't link).
Needs to be in by end March for secondary (I submitted mine early).
Google your local authority and "placing request".
It's also worth talking to the school you want to go to. I talked to the school I wanted ds to go to, so knew that to date they'd always been able to accommodate all placing requests and, more importantly, was able to get him involved in transition activities that they did for P7 kids in November.