Go on to the O2 website, ring them or call in to a store and ask for the text anytime. I am assuming you are on pay as you go. For £10 a month you can get 300 texts, £20 500 or £30 1000 texts. You can also nominate a number as your favourite number to get 500 minutes free. I have this and only spend £10 a month on my phone on O2 PAYG.

Hope this helps.