In AmE porters at airports are called 'skycaps'. They're usually found at the curbside check-in counters where passengers are dropped off.
Just make eye contact with one of them, and they'll usually offer to help with your luggage. Otherwise just wave at one and ask "Can you help me with my luggage" or "Could I get some help with my bags, please?" They'll of course expect a tip and possibly a fee. Since I've never used one, I can't say how much they cost or what the normal tip amount is.
You can usually rent luggage carts for a fee as well. Many airports have them in self-service racks for a fee. Again, I've never used one, but I think they typically cost something like $2-3 dollars, but don't quote me on that.