Texting a girl is nearly no different from texting a friend of yours who is a guy. If you are friends with the girl, there is no need to feel the pressure of texting or of sounding creepy if you are simply only being friendly with her. Different methods of texting are possible, depending on your current situation and your relationship with the girl you are texting. If you are interested in the girl you are texting, various ways of approaching the subject are available without sounding creepy or pushy.
Send a casual text to see how she is feeling before you decide on what you would like to talk with the girl about. If she responds quickly, she is more than likely available to talk. If she responds later or the following day, you may want to see further how she responds to casual texts (such as saying hello or asking her how her day is going).
Text the girl you are interested in only once and wait for a response. If she does not respond to you within a week --- try once more. After that, she may not be into you and it may be better to pursue or talk with another girl.
Avoid spam messaging a girl's phone, as this may bother or annoy her. Instead, give her a call when finalising plans or condense your message into one text rather than sending multiple text messages repeatedly.
Keep casual when making plans and avoiding asking her on a romantic dinner date with a text message. Asking her to see a movie or grab a quick bite for lunch is acceptable if the two of you are already friends and it is a casual outing.
Avoid coming on too strong by saying "I love you" or other romantic sayings in a text message, as this does not give you a chance to talk with her in person or view her reaction personally. Wait to say important messages in person to keep her from avoiding you and be sure it is at the right time in your relationship (where you both feel comfortable with the pace).