losing weight
 

my current bike has a steel stem and handle bars, would it be worth switching them to aluminum to help lower the weight of the bike?

It depends on how much you're willing to pay per pound.
You didn't mention what bike you have and what you use it for. If you have a fat tired MTB and use it on pavment only, skinnier, slick tires would be the way to go first IMO.

"would it be worth" is impossible to determine without context.

Maybe, maybe not.

it is a full suspension mtb. i will use it mostly for road use maybe light trial when i get more experience. i am 40yrs old and don't think it would be a good idea to try drops and jumps knowing i have to go to work. also it has an aluminum frame with a rear steel swing arm.

I agree Bill Kapaun. If money is an issue or you can do it in parts the first thing and cheapest thing you can do is go with skinner (or narrower) slick treaded tire and a lighter weight tube. The next biggest gain would be to go with a set of lighter wheels that can accommodate even skinner tires. Your largest performance gains well be felt the most by reducing rotating mass weight.

Then after that's been done then worry about the small stuff like your handle bars, stems, seat, seatpost etc. BUT I have a feeling that if you have steel bars and stem then your frame is probably very heavy as well. If that is the case then upgrading to lighter stuff may become a waste of money with little being returned; thus a new lighter bike would be the better idea. If you can't afford that yet but planning in the future to do that, then get the tires and maybe the rims too if you can swing both, and forget the rest.

Chances are the bike has something like 13/14-34 cogs and a large chain ring of 42-44 teeth.
With the skinny tires, the bike will be so much easier to pedal, you'll probably end up "spinning out" in high gear. Then what to do next....:)