Don't worry about the grammar. You can hire an editor for that. A proofreader can also do the job of finding plot holes for you. Now, zombie apocalypse novels are a tough genre. You need to research all weapons be it melee or firearms, etc. You also need to be realistic as it was a modern survival thing. I can see that the story is going good so far and it has potential. The direction of the story is also superb! Good thing is, the characters are lifelike. I hope you improve your craft shortly. Will wait for that time to come ?