First of all, you should check the Book Review guidelines. Then you need to type what you want to post. From personal experience, you might not get enough but it helps if you keep the book near too so you can remind yourself what things are you forgetting.
As for not spoiling the plot, try to move around it. As if a character dies, present it as '... where fate deals a dark hand at one of the characters. ..." It doesn't matter if it sounds too revealing because since you have already read it, you know what it means.
As for getting others interested, try expressing why you like the book and what you think will appeal others.
As for explaining characters, I am not much help but will say that you present them as someone everyone can identify themselves as (or can be lured by the mysterious description.)
Best of luck,