I think that EE SIMs are nowadays supplied already active, and this seems to confirm it
http://ee.co.uk/help/getting-started...ating-your-sim
I'm not sure that there is credit on there. Probably not. I think the £10 on the packet is about a monthly bundle option pre-selected.
You can set up an online account and check these things, alter the bundle if you wish, monitor amounts used and remaining (there may be some bonus free data to start with), and so on. I don't think you'd able to top up online with foreign debit or credit cards, or this certainly didn't use to be possible, but you can pay in shops to add credit directly or buy vouchers
If you do want to change the monthly bundle option, do it before adding credit.