More info about tying up plants, with a short video
Side shoots are easy to spot, as they normally grow out between a leaf and the stem. However, as shown in this photo, this side shoot is growing straight out of the stem. It is easy to identify as it is thinner than the main stem, so just pinch it out with your finger tips or a small sharp knife as close to the stem as possible.
More info about removing shoots
Twin Headed Plants - You will find that some tomato plants will split into a double head, in this case just cut out the weakest one, or if in doubt leave it a week or so to see which one is the stronger head.
No Head On Tomato Plant - You will also occasionally find that a plant suddenly has no head, so you are left with just a truss. In this case you will need to let a lower side shoot take over as the main stem for the plant. We call this Growing Blind and I do find it happens more on beefsteak tomato plants. Here is an example below, which luckily shows a nice shoot which I can use as the main plant.
I have also tied up most of my tomato plants this week, box cord is best as it rots in the compost heap and is slightly wider than polypropylene string, so will not cut into the plant with a heavy load later in the season. Some people use poles to support their plants, but with such a heavy crop on them they will slide down the poles.