Cutting tomatoes isn’t exactly rocket science, but there is still a hardway and an easy way.
By far, the best knife in your block for cutting tomatoes is the small serrated-blade, also called the “utility” knife. (About $12 at Kohl’s)
Using fresh tomatoes helps, as well. The older a tomato gets, the tougher the skin, and softer the insides. When combined, this can turned “slicing” into “squashing.” This is where the utility knife really earns its keep, slicing through the tougher outer membrane without enough force to smash the fruit.
The serrated blade is what my the knives in the old Ginsu commercials famously able to “cut through a Coke can, and STILL slice a tomato!” 😉
If you don’t have one, get one. It’s a great knife.