How to tell her you love her?

Girls love decisive guys, if you are undecided about your love for her it will show in your words. So be clear about your emotions as the first step.

A few things you should avoid doing, when you ask her to be your girlfriend are given below

Being timid doesn't work
Don't ever say something like "I think I am in love with you, would you want to be my girlfriend?" It's almost a guaranteed reject and it sounds very servile.

Dont go out on a dinner
Don't make this into a special date. If you take her out for dinner and then propose your love to her, she might reject it and then the dinner becomes awkward and embarrassing. Worse, she might just walk off on you. Avoid yourself the pain.

No indirect communications
Don't send her an email or worse an sms.

No calls
Don't call her up, it is difficult to convey your feelings over a call and you cannot judge her reaction just by her words.
All the things mentioned above are to avoided because it either communicates a timid attitude or is not direct enough.

Here is what you should do
Its best to ask her directly and out of the blue. Use the surprise element in your favor. It does not matter where you ask her, just be frank in your words.
Say something direct like "I am in love with you and I want you to be my girlfriend". That's it, short and sweet. Don't say a word beyond this and maintain your silence while looking deep into her eyes.