This is an awesome question but the subject of cancer starting is really complicated and involves lots of different things.
Cancer is a group of cells which grow uncontrollably in our body. The, cells divide and grow uncontrollably, forming tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the body through the bloodstream.
Finding out what causes cancer is really hard. Many things are known to increase the risk of cancer, including smoking, some infections, radiation, lack of physical activity, poor diet and obesity, and environmental toxins. These can directly damage genes in our cells and eventually cause cancer.
Cancer usually first develops when you get a number of genes in a cell which are damaged. You normally need at least 4 different genes damaged. But its not just any gene, it has to be a specific combination of genes involved in cell growth, multiplication and cell death, along with other things.
The reason its so complicated is that there are hundreds of genes that can help cause cancer when they are damaged. There is also lots of combinations of these damaged genes that can cause cancer. So there is lots of way you get cancer…. it depends on the genes damaged, the combination of damaged genes and whats damaging them!
More info- besides external factors that cause our DNA to mutate, our genetics can also be a big factor that contributes to cancer. For some of us, our genetics or variations in our DNA may contribute to the process of cancer. However, the effects of these variations may be small and it might take the process of a life time before those genetics come into play. In other cases, people can inherit DNA variations that have a big effect- sometimes you see families where a cancer frequently occurs in members of every generation (ex. melanoma in a great grandma, grandad, mum, daughter fo the same family). In fact, there are some DNA variations (usually pretty rare) that results in almost a 100% chance of developing a cancer in your lifetime. This is becuase the variation you have inherited is basically in an importnat gene involved in cell growth and usually has a detrimental effect on the protein that is produced. In this sense, the cancer technically first begins to develop based on the predisposition of your genetics (or variation in our DNA).