Select the cells that should be locked and then use:
format|cells|protection tab|check lock

Select the cells that should be unlocked and uncheck that setting.

Depending on the number of cells of each type, it may be easier to lock (or
unlock) all the cells and just unlock (or lock) the others.

Then you have to protect the worksheet
Tools|Protection|Protect sheet
(give it a nice password)

Be aware that this kind of protection is easily broken and that there are lots
of things that are unavailable on protected sheets.

You may want to do some testing to see it still fits your requirements.

You don't need to create a template file (*.xlt).  You could always use an
existing .xls file and just do a file|saveAs when you want to start a new
workbook.

But you can create a template if you want.
File|SaveAs
And choose "Template (*.xlt)" from the "Save as Type" dropdown.