1)Purchase 3DS Max/Maya/Cinema 4D
2)Learn to model, texture, and properly animate CGI in a professional manner
3)Model, texture and animate a model of an exploding car, along with particle effects to add that extra bit of "realism"
4)Render it out overnight (or for days, depending on the complexity of the scene and your PC/Mac specs), then use Apple Motion/Adobe After Effects to composite the rendered CGI into your live-action footage
Judging from your post, it doesn't sound like you know the exact steps of creating CGI for a movie. It takes time, patience, SKILL, computer power, time, patience, MONEY, time, time and time, and talent. CGI is one of the most complex parts of filmmaking and isn't exactly easy for an amateur filmmaker to take on (I speak from experience here!). However, I wish you the best of luck!