I’d say to start by cutting out different types of other meat slowly. Don’t go straight into pescetarianism coz it’ll be hard to adjust.
Although tuna and chicken taste really different, they are generally interchangeable in recipes/meals in the way that they taste with the other ingredients.
I’m not the most committed cook, so I don’t have too much to offer in terms of recipes! My staples are fish steamed or baked with lemon/herbs. Teriyaki salmon also is surprisingly tasty. Just cook salmon any way you like with the teriyaki sauce.
And if you don’t love sushi, learn to love it! Pescetarian staple! :D
Good luck! x