Declare your intentions
The most common mistake guys make is by becoming best friends with a lady they really like then later try to turn the relationship into a romantic one. Trying to transform a platonic relationship into a romantic affair is utterly quixotic; it only happens in the movies. Let her know from the start that you think of her as a potential
Let her know from the start that you think of her as a potential partner, not a sister. This means when she begins blabbing and seeking your advice on some other ‘cute dude’ who is eyeing her you immediately interject “Sorry I’m not going discuss other guys with you. Let’s keep this conversation between you and me!”
Even when you are sure she is the one you would wish to spend the rest of your life with never reveal this on the first few days. Proclaiming that you want to marry and have 5 kids with her on the first date will only scare her off especially if she isn’t interested in settling down just yet. Drop subtle hints that you are interested in a possibly long term relationship without much detail.