@lucysmam I haven't kept mantids but I have kept cockroaches (yes, you read that right!), and honestly, it is quite easy to keep them as long as you provide them with the right heat, humidity and food.
Enclosure wise, it depends on what variety of mantis you're going for, and what instar (the age of the insect) you are going for. If it's tiny/a baby, don't get a big cage as it might escape or be difficult for you to find in the enclosure. It will need the correct substrate - I used coconut based substrate for my roaches, but I think mantises prefer soil, but you'll have to double check. Get some bark/twigs for it to climb on. Get these from pet shops and not from the garden, as they may have other insects or bacteria on that might kill your pet! They like to hide and they hang from them when they shed.
You will need a heat lamp or mad, as well as a thermostat and a humidity gauge. Before you get your mantis, make sure you can get your enclosure to the suitable heat and humidity levels - again, this depends on the breed you choose! - and a spray bottle so you can spray the cage down to help build humidity if it needs it. For decoration, you can get fake plants and flowers, which the mantis might like to hide behind.
Generally, insects are easy to look after, but you must make sure that the temperature and humidity are looked after at all times, otherwise it will die. They're just not built to deal with our climate, especially as we head into winter! Also, as a final note, the mantis wont get lonely! They are used to being on their own. As long as it's fed and has somewhere to hide, it will be happy! Don't get it a friend, as they will probably fight and eat each other.
Anyway, I hope that helps. They're fairly simple creatures, but very interesting and fun to look after, I have been told!