You mean you just joined 48 and can't get connected? Is the SIM activated? Is the phone unlocked for use on any network? Is it an iPhone? If so then a common issue is due to the stupid way that Apple do things. iPhones need to activate on certain networks and on some this involves the phone "silently" sending an SMS message to a UK activation server. On 48 international texts cost 25c each charged to your account cash credit balance. So it's possible that you need to put some cash credit on your account for this to work, reboot the phone and wait a while to see if it activates. If none of that helps then you may need to contact 48 support for more advice. See the link in my signature below.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.