During school time, we just studied. You just need to start. No need to invent something if you are doing it for yourself. Personally, I just became engaged in armwrestling in a year and a half, but there was no such goal. Just exercise.
About nutrition, and about sports, it is much more effective to listen to or watch lectures on the Internet, it will take less time.
In sports, like any other area of life, the main thing is start with the little things.
1) Replace unhealthy foods (if you eat such) for other foods. It is not necessary to immediately give up something, you can gradually reduce the amount. Fast food, chips, chocolates, sausage, salt, sugar, flour products. Find alternatives. Can't drink tea without sugar? Add honey or substitutes. Are you very used to mayonnaise? Take sour cream. Etc. + drink plain water instead of any other cold liquid and try to use fresh vegetables as much as possible
2) Sports should not strain you, cause you a feeling of discomfort, laziness or a desire to stop it. Start with something simple like a 5-minute charge in the morning. Even such a small detail can gradually instill a more positive attitude towards physical activity
The book at the beginning is unnecessary. The book is primarily a tool for making money for a publishing house.
I advise you to read on the forums (for example, a stylfactor or running), there are many topics similar to yours. If it is not clear, ask there, although athletes are strange people in the majority, they will not be too lazy to give advice and help.