If you are approaching a stranger, avoid an aura of desperation - women can sense it from about ten miles away, and they will avoid you from the moment you open your mouth.
Relax, be friendly, and smile. Don't try and keewp her talking for ages. If you see a girl you like in the same place at the same time, you can start with a simple 'Hi' and a smile. In a day or so, try a little conversation about where you are - park or what ever. Ask her if she fancies some company on her walk - you will very quickly get a vibe if she is interested, or not, and if not, then retreat gracefully.
Remember, for a lot of women, being approached by a strange man is not a comfortable situation, so you need to take things nicec and slowly. If she is interested, she will respon, if not, keep your dignity, and return to a smile and a nod the next time you see her.
Good luck.