Are you looking for crystals for focus, to help with the cancer, energy support or all of the above?
For focus, amethyst or fluorite would be a good choice, they assist with mental clarity, these are easy to find and easy energy to work with especially if you are new to crystals.
You can also use selenite which assists with energy flow, mental clarity, and we've had some good results with selenite and cancer too. Chlorite included quartz also helps fight cancer too.
For stress and mental clarity, amazonite works well, it helps with pain management too.
For energy, citrine is a must, rutilated quartz also boosts energy levels. Smoky quartz will assist with sleep and help you keep balanced. There are also many stones that will help with addiction too.
I work with crystals extensively every day so if you need more assistance, just let me know. (I have an online metaphysical crystal and mineral store, the link is below).