Learn how to cut a cake in layers so you can use fillings in between each layers. I will teach how to

do it just by using a serrated knife. 
There are tools that are sold just to cut layers on a cake.The best one out there is the Agbay, but if you can't afford the price the price of it, just a good quality serrated knife will do.
If you still rather prefer to buy a tool, make sure it's ofgood quality. You will find a few different ones in the market that are less pricey but you might have to test them to see if they work well for you.