I started wearing dresses when I was a latchkey kid during my junior high and high school years. I had realized in elementary school (if not earlier, but I can't remember farther back than elementary school) that girls could wear dresses and skirts but boys could not; I had also started thinking girls were lucky because they have many more choices in clothing they can wear.
Once I realized that being a latchkey kid gave me the freedom to actively pursue new interests without fear of being discovered by anyone, as well as the ability to easily pursue wearing dresses and skirts at the home I shared with my parents - and it didn't take me long to realize all this - I started going through my mom's closet on most school-days once I got home from school, picking out one of her dresses (or one of her skirts & one of her shirts or blouses) that I thought looked pretty, and changing from the boy's clothes that I wore in public that day to that dress (or that skirt & that shirt/blouse) of my mom's. I didn't change back until just before the regular time that my step-dad would arrive home with my younger sister.
I never left the comfort zone of my parents' house (out of fear of being embarrassed and seen by someone) while I was dressed up like this. So I very much enjoyed this time alone pretending I was a teenage girl or a young woman.
My advice to sisi would be to truly pursue these feelings you had when you were helping that customer choose a dress for his 5-year-old niece. If you fear being discovered, sisi, then do this when you have some time alone.
I think Step # 1 for any man or teenage boy who is just starting to pursue wearing women's (or girl's) clothes should be to do a little research into how clothing for females is sized. This is because clothing for males is sized in a pretty intuitive/easy-to-understand way; but clothing for females is not. One way to do this is to go to the website of any clothing store for females and look for their sizing charts (virtually all such web-stores have a page devoted to this). Another way to do this is to go to a brick-and-mortar store that sells clothing for females, find a customer service (or sales or something like that) employee there, and ask her about how to figure out your size in women's (or girls a.k.a. juniors) clothing. I have found that virtually all of them will be happy to help you.
Then Step # 2 would be to browse through that store's selection of dresses (and skirts, if you're also interested in those). Find one of those in a design that you like/a design that you think looks pretty. Once you've found this, look through that store's selection of clothes in that design, checking the size of each one, until you've found one in your size (which was determined in Step # 1 above). Finally, take this article of clothing to the store's register/check-out location. Again, I have found that virtually any employee that you get at the location to pay for the article(s) of clothing that you have selected will be happy to assist you.
That last paragraph was for a brick-and-mortar store. The equivalent process for a web-store should be easy to figure out.