By default all cells in excel are protected or locked, select the cells you
want to unlock and go to format, cells, protection and uncheck locked, then
go to tools, protection, and protect sheet, enter a password if you want,
now only the cells that you unlocked  can be edited. Be aware that this
protection is very easy to break, the code to do so can be found very easy,
but it will work for most people .

If you only need a few locked I would select them all first, Ctrl A, then
go to format, cells, protection and uncheck locked, then select the cells
you want to lock and go to format cells and check locked, the go to tools,
protection, and protect sheet, enter a password if you want, now the cells
that you locked can not be edited