When I moved from Pakistan, I was told by many that the only jobs I can get is working in a shop as graduate careers aren't for us. The fact that I didn't take their word made such a difference. Self-belief is the first part of the equation and once you have that, top it up with doing your research and acting on it as actions make the most difference. There is a lot of material available for applications and what it takes to be a civil servant at HMRC. Have some time to read and understand it before applying. Lastly, overthinking is not your friend, so my advice is to do your thinking but DO NOT over think and take actions to make a plunge!

Always be yourself, it you make a mistake then admit it, we are only human. Listen to the views of others around you. Treat staff how you would like to be treated but bear that everyone is individual and treat them accordingly.