As your tomato plant is growing you have to keep an eye out for suckers...which are not bugs but small branches that are trying to join from an existing branch. So, how do you find a sucker? Here is a picture of one.
Suckers suck the energy out of the plant therefore taking the nutrients away from the plant. The best way to get rid of it is to pinch or snap it off. It allows the plant to grow taller and produce more tomatos which are better tasting!