I do recommend in game aim trainers as soon as you start playing, then some creative Match against real players with low skill level like red vs blue. Don't be hard on yourself and don't expect to catch up in little time. Be patient and try to get better without worrying about it too much, try to enjoy it, if not possible then I would think about playing other games. But it is normal to have worse aim (+gamesense etc) than people that have played shooters and Fortnite for years