This is correct. Devotion to a saint or to the Blessed Mother does not take away from devotion to Christ. Rather it enhances it.
I think of the saints as being friends and role models. If you were trying to get the attention of the president of the company you work for, you could go directly to him and maybe you would, but you would normally also contact any friends you had who were vice-presidents of the company and tell them you had this important matter and could they put in a good word for you with their boss. Saints are like that.
Plus, saints come from all different backgrounds and they served God in all kinds of different ways, so you can pick one that you personally relate to the most because s/he had struggles like your own, had a career or a family life like your own, was from the same area as yourself, etc. My favorite saints all teach me through their examples certain things about how to serve God better or serve Him in certain situations. And time spent reading their writings or thinking about them is likely more productive, spiritually, than reading up on some Hollywood celebrity.
With respect to your second question, “how to show devotion” and “which saints”, usually I just ask a certain saint to pray for my intention, and I might go to a Mass on my favorite saints’ feast days or light a candle in front of a statue of them at church. As for “which saints” you can pick any one that you like. Certain ones are identified as patrons of something specific or of a specific group of people (for example, St. Peregrine is the patron of cancer patients, St. Anthony is the saint for finding lost articles, St. Joseph assists with selling a house, St. Matthew is the patron saint of civil servants, St. Andrew Kim is the patron saint of the Korean people, etc. ) so people with those specific needs might turn first to those saints, but you don’t have to, you can pick any saint.
