Firstly, if it's brand new, bring it back or send it back if you bought it online, because it'll be on warranty for probably a year.
If you're trying to trim long grass, try not to push the trimmer too far into the grass as it can overload the machine and tangle up the head.
Is this a petrol/gas machine? If so you need to use two stroke oil at a ratio of 1 part oil to 50 parts gas to lubricate the machine and stop it overheating and seizing up.
If the trimmer is electric, is it suited to your voltage supply? A 110-volt trimmer used on a 240 volt supply will overheat and burn out or if you're lucky an overcurrent circuit breaker device on the tool will cut off power before this happens.