Trust me on this. First I want you to understand that 20 minutes a day is nothing for getting in shape---it wont happen. Second, you can not teach yourself boxing by watching a video or reading a book. Third, you are a long way from punching anything, or investing any money in any kind of bag. (I know fighters in training that use their bed mattress proped up against a wall for punching). The end result after several months is that you will be out of shape, you will be no where close to even getting in a ring and sparring with anyone. And worst of all, when you finally do decide to get gym membership and learn the 'sweet science', your trainer will spend weeks having to undo all the wrong things you have picked up.
If your funds are such that you can not afford to get into a boxing gym, then go by and ask them if you can work in the gym in exchange for free lessons. If not, then ask if you can hang around and watch. You can pick up a wealth of training tips by just watching.
Ask some of the fighters if there is a retired boxing coach who might just be bored and would be glad to donate his free time to working with you. You can cut his grass or do odd jobs around his home for payment. I am doing that now. I am 73 and got tired of being expected to be at the gym on certain days and spend hours of my volunteer time teaching. I now work with a young fighter who was adopted by a family, and it has put the fun back into training someone...and I have a nice lawn and trimmed hedges.