The main stem is the one the flower is coming out from. Look above the flower, the big leaf stem going out to the left is part of the main stem, and that smaller leaf stem just at the top and slightly right of that is the sucker. That is what you would/ could pinch out. Some people leave the suckers on; this will produce lateral stems and branches and will eventually produce more fruit, but it is also more work because you have to tie each stem to the stake. I'm racking my brain about Early girl: isn't it a determinate? With determinates it's not as important to pinch suckers as it is on indeterminates because determinates usually reach a set height and quit growing. If this were my plant, I would let the growth get a little bigger, then pinch out the sucker. This is easily done with the fingers. Also it will become more apparent to you which is the sucker. I've been growing tomatoes for 20 years, and I pinch out suckers at the micro size stage, but I'm familiar with where they are. Suckers almost always come out between a leaf stem on the main a ranch and the branch itself, often right above a fruiting stem.
I placed crosses on the picture that's what I think to cut based on your feedback. 
