How can I ask out a girl I've seen for awhile?

well i have noticed this girl in walmart in my home town. and she is really pretty. and i have seen her for a long time now. i know that we have locked eyes plenty of times before and i have went up to her before but only for wire transfers. i really like her. and recently i went in to walmart and we locked eyes right away and i wanted to smile but i just couldnt do it. i feel like im gonna scare her. i dont want that. but i wanna smile at her becuz she beautiful and im attracted to her. anyway i went and made my purchase and i was mad becuz i wouldnt get anywhere near her station when i head out. the door is closer. and i really wanted to try and speak to her. or even smile. but when i turned around she was right there at the door waiting. she was right there and i couldnt believe it. i walked and i didn’t evn look at her. i wanted to and i had the best opportunity to ask her out while she was alone atleast. she was right there like maybe she likes me and wanted me to talk to her. but who knows i cant assume that. it most likely was a coincidence. but ill never know. i plan on going in there again soon and i have planned on finally approaching her and talking to her. i think i wanna just ask her out for lunch sometime to hang out as friends and i do mean friends. i wanna do this slow and i am really determined to do my best to make it work. but i have to ask her first and not get ahead of myself now. i have major confidence issues and low self esteem and i have never dated a girl before ever and im 20 years old. i get made fun of all the time at my work i act like i dont care but i do. this girl is very attractive to me and i wanna give it a try and go for it. any advice for me about other things i could do or if i am doing anything right her or going about this the right way. let me know thanks.