>I asked her if we could go out some time and she said 'Yes'

>I need a advice when to ask her out

You already have asked her out - just to something very informal, hanging out in the park, and she said yes and has invited you to her house. You can ask her to another date pretty much anytime. Say that you had fun the other day in the park and that the two of you should do stuff like that more - from what you've said, she will agree with you. Then just suggest something; a movie, a drink, whatever, but maybe something a bit closer to a date than just sitting in the park (e.g. don't take her out to dinner at a fancy restaurant, but do do things where its just the two of you in a situation that could be seen as a date - go for coffee etc)