1. If you just want to write well, the quickest way is to take a 68-day calligraphy correspondence course in Xi Shu.
2. If you want to write regularly, you should learn Pang Zhonghua's hard pen calligraphy. Xinhua Bookstore has copybooks that can be copied everywhere, so you should stick to them.
3. If you want to achieve something in calligraphy, the introductory textbook is "One Hundred Days of Calligraphy", where you have made a detailed explanation of Cao Chuan's regular script, which has benefited people a lot. Then, as needed, specialize in the corresponding book style.
The above is for reference only. Just stick to it for a few years! Wish you success!