Okay, it sounds as though the main thing behind this "school project" (from the viewpoint of you your teacher/lecturer) is for you to think about it,use your brain and learn something. If we do your assignment for you, then this doesn't happen.
From what you have told us.
1) You have a 12 volt battery
2) You need to provide a regulated 12v output with a current capacity of 5A
You need to consider the following.
1) The terminal voltage of a battery may be higher than 12volts when fresh.
2) The terminal voltage of a battery will fall when the battery becomes depleted.
3) A linear regulator such as the LM338 or LM7812 will require a higher input voltage to produce a 12v output. This will usually be somewhere in the order of 2-3 volts higher than the output voltage. So, the easy way isn't really an option. Which is probably why you were given this project
This Link might give you a start on batteries. Remember ... Google is your friend.
We can't do your assignment for you (that would just be wrong) but we might be able to point you in the right direction. I would suggest that you start looking for a switched-mode boost controller. It should be easy enough to design a small and simple Power Supply which could provide a regulated 12v output with 5A capacity from your battery. I would look to design it with an input range of between 8 and 14 volts. Using currently available components, you are probably going to need a simple switching controller, a coil, flyback catch diode and possible an output FET along with the odd current sense / limit resistor. You should be able to find all the info you need at the
**broken link removed**