Changing the relationship with a parent is a difficult but key step every person takes as they start to become an adult.
Your mother is well aware that you will become interested in having a relationship eventually. (Remember she went through the same feelings as you at one point).
Perhaps she won't be ready for your news, but perhaps she will. Regardless she will appreciate that you still want her to be there with you as you begin this journey into adulthood.
I would talk to her, over a shared meal or do a walk together. Something that creates an intimate and comfortable setting.
Then you tell her you have started becoming interested in boys and have questions about relationships. Expectations insecurity etc.
You will have a good feel for how things are going, it might be best to reveal it slowly over a few weeks. Worst case scenario may be very unhappy about your relationship, but whatever her response, she will be glad if the reminder that you do care and listen to how she feels.
Its your life, but you she is still an important part of it.