From the living room to the hall where the carpet ends I need a new metal divider,seen the one I want but what do I use to cut it to the right length?what tool?a hacksaw?

Hacksaw would be find just make sure it's a blade for cutting metal, the teeth on the blade will be small for metal cutting.

Yes hacksaw will do the job, make sure you have the right divider for the doorway because if you have carpet in the hall you need one with two open edges for the carpet to slot in from both sides.