How to buy plan for a sim I got from the local store

I picked up a sim from my local phone store. I go to Lebara and it says is this plan for an existing sim or a new sim. If I select new sim it will send me one in the post which I don’t want because I’ve already got one. If I select existing sim it asks me for the number, which I found out by dealing *#100# . I entered the number and it said it send a text with a verification code but that can’t come through because the account is not active.

any ideas?

renzo125 you can just activate the new SIM following the instructions on the packaging then select the plan you want from the website using the existing SIM option.