Yes definitely! Communication is key in every relationship. If you don’t enjoy sex express this immediately, even during sex, if there is something you don’t like, communicate it. Your partner cannot know if you don’t tell them. It is super important that you are able to be open in your relationship and express what you feel. Share your feelings, share what you like, what you don’t like, what makes you feel awkward or scared or what hurts you and makes you feel uncomfortable. Your partner is there to cherish you and love you. If you don’t communicate this they won’t know how you are feeling!
In your question you ask how should you tell them. There is no perfect way to say how you feel and if you are not used to expressing how you feel you may experience lots of different feelings coming up like anxiousness, stress or worry. Sometimes the more we leave things, like expressing how we feel, the more these feelings and emotions build up and seem bigger than they are.
So don’t worry about how you say it the most important thing is saying it. If your partner is not open to listening to your feelings maybe reconsider the purpose of your relationship and why you are in it .. a relationship should be a space of mutual growth where you feel cared for and safe to express what you feel.