More information is needed. Would this be your first ever girlfriend? I don't advise putting all your eggs in one basket with one girl; if she's a friend of yours then it could be the textbook "friendzone" scenario. Everyon knows how that goes down. However, if she is still virtually a stranger, then there is nothing wrong at all in asking to go for coffee. To make yourself more appealing, always be comfortable in yourself. You don't NEED a girlfriend to be happy, same with girls with men.
As for how long to ask her to be your girlfriend... that all depends. You certainly woudn't ask her straightaway. My advice would be to go on dates, keep them fun, and if things are going well in a few months, then that would be a good time to call yourselves an official couple. Good luck my dear.
Age.. 31. you should know that you never 'confess' . You seduce her.. you have sex with her and you let her open up to you emotionally. You reciprocate.. Boom she is attached to you and you become exclusive. You sound like you have the mentality of a 15 year old.. 'confess' comon dude grow the fck up.