You would certainly be welcome at Mass and no one should tell you off at any point. Plus no one will know you’ve been away unless you tell them.
Buy a copy of the text of Mass (as it changed slightly in 2010) and look through it beforehand so you know what to expect, what to say, and when to stand, kneel etc.
Would you prefer to a quiet Mass or a busy one? If you prefer quiet, go to Mass on a weekday or Saturday or Sunday evening. If you want a busy one, try Sunday morning.
Do you want to be recognised or incognito? If incognito, you could go to Mass at a church not where you live, but perhaps near your work, or come to the Cathedral.
Look at the websites or noticeboards of local Catholic churches for Mass times.
If you want company at Mass ask someone you know who goes to Mass if you can go with them (when and where they next go to Mass), and sit with them during Mass.
If you know a priest who you trust, talk to him beforehand about any concerns. If you have something you want to ‘get off your chest’ first, ask a priest if you can come to Reconciliation/Confession, and if he could guide you through that.
Material produced in collaboration with the Agency for Evangelisation in the Diocese of Westminster.