If you have an issue or concern at work it’s almost always better to raise it informally by having a quiet word first. Problems can often be resolved this way before they get any worse.
You may feel nervous about raising an issue, but many employers would rather resolve problems quickly without having to go through a formal grievance process.
Choose someone who you feel comfortable talking to. If this isn’t your line manager ask to talk to another manager or someone from HR.
When you tell them about the problem they should arrange a meeting to discuss it as soon as possible.