tonker,
If this is your first flying job, then you can't legally be a Number 1 until after 12 months of flying...that is a CAA regulation. However, i may be completely wrong and just dreamt that up!!
You will know when you feel ready to go for it, everyone is different and some crew never want to go for it....However before you do apply, just ask yourself if you could confidently deal with an emergency on board? Being a no1, the crew will have all eyes on you to wave that magic wand