I can create explosion like the one in starter content pack but it doesn't look that realistic and also I wanna create large explosion so how can I do that? Something like in Call of duty. link to Tutorials would help.
You'll have to learn to use the Cascade Particle Editor. Unreal Engine has a tutorial series on Youtube to get to learn it. From experience, jumping in to the engine and trying to make something like a realistic explosion won't be easy. I would suggest taking a look at the starter pack explosion to get an idea of what to do. Also, StackOverflow is for specific programming questions, not like this one. I would try over at UE4 AnswerHub for broad questions like this.