You wouldn't have the got the job if they thought you were not suitabley qualified..I am sure you are doing a fine job :)
Don't undermine yourself - be proud that you are working - you say you have only just started there so it will take time to settle in eh?
I am not sure what sort of job you are doing..but if you are unsure always ask questions..then you know in your own mind that what you do is correct.
Look around for friendly faces among work mates, try and make conversation even if it's only about the boring old weather at first lol. Once you have made a couple of friends you will be fine.
Keep up the good work and let us know how you are getting along.