Now that you have a girl's phone number, you might want to play things safe by texting her instead of calling right away. Text-messaging is an acceptable way to communicate simple messages and random, flirty thoughts without having the feeling of vulnerability that voice-on-voice conversations give you. There are, however, some things to keep in mind when it comes to texting a girl so that your text messages are not perceived as awkward.
Choose your timing wisely. You do not want to text a girl five minutes after she gives you her number, or this will come across as overly eager. Unless the two of you established the need to text the same day regarding a homework assignment or work project you are doing together, wait at least one day before texting her to avoid any awkwardness.
Text her after work hours. Texting a girl in the middle of the day -- when you are supposed to be working or attending class -- makes it obvious that you are thinking about her. If you text her in the morning before work or school, this is even more awkward. The best option is to send a friendly text in the early evening. Additionally, avoid texting her late at night, because you don't yet know what her evening routine is or how early she goes to bed.
Have a cheery tone in your text message. Say something funny to make her laugh and smile. Keep it light, simple and friendly. Avoid getting overly personal in your text. Do not text her to confess that you had a dream about her, or that you miss her after not seeing her for 24 hours, as this may make her uncomfortable.
Do not become emoticon-happy. Too many smiley faces, frowns and other facial expressions can make it seem like you are trying hard to get her attention.
Avoid bombarding her with text messages. If you send a text, wait for her reply. If she doesn't respond, do not text her again asking for a follow-up, or wondering what happened to her. This looks needy and can become uncomfortable real quick.
Know when it is appropriate to pick up the phone and call her. If you made plans to get together but need to cancel them, you should not do so in a text message. Cancelling plans is best done in a live conversation; it's a more thoughtful way of communicating. Also, if you have a lot of information to give her, do not put everything into a text message. Save the sharing for when you see her in person.