It already comes with two keys to override the electronc lock, so the feature you're trying to add already exists in the product... If you're set on doing this anyways just to be able to open the safe with your computer you'll first have to disassemble the safe's electronics so that you can see what's going on inside. I'm sure it's some very simple solenoid which does the actual work which would be extremely easy to interface to a computer with. Get the thing apart, take a lot of decent pictures and post a link or the pictures directly here so that we can take a look at them and it should be a fairly simple project.
Another good place to start is purchasing one of the recommend begginer electronics books that users here recommend, I think there are some sticky threads for recommend noob reading material, and even a bad book is going to teach you something if you keep plugging at it and asking questions here when you're truly stumped. There are ungodly amounts of only resources on beginners electronics, remember Google is your friend. The only book I've ever read on electronics is "Teach yourself Elecricity and Electronics 2nd edition" which isn't the greatest (so I found out afterwards) it's a little dated and focuses a bit too much on the obscure math and teaching circuits (a circuit if built wouldn't actually do anything) But it teaches enough of the basics to get you started (and it always helps to have a real paper book to sit down and read)