Start with chicken. It will be easiest for your guy to transition to. After about two weeks, you can start introducing new proteins a bite at a time for a week or so until they adjust. You also need to make sure they are getting plenty of bone. The ratio most used is 80% meat, 10% bones, 10% organs (5% of which should be liver). At first, your guy will likely need more than 10% bone so that is another reason chicken is a good idea since the bone content is high. You can adjust the amount of bone to fit the poop needs. If the poop is not solid, add bone, if it is too solid or chalky coming out, then you are giving too much bone. It is so easy to adjust from day to day. Just like the amount you feed. If you notice he is getting fat, then cut back, if he is getting too thin, add more. It won't hurt them while you figure it out. I added organs pretty quickly for my guys because we had a pressing need (health issue) but normally you can wait a few weeks and then just start with a smidge and work up. Some don't like organs much so I have found that if I freeze them, they will eat them. Also keep in mind that the bigger the better on pieces of meats unless you are feeding no bone then chunking is ok. Anything too small might be a choking hazzard if it has bone in it. For a pup, I would do leg quarters, thighs, bone-in breasts or offer the half chicken and take it when you feel that they have had enough. You can then re-offer it at a different meal for them to finish it off. You will also need to check the nutrition info on the back of the package. Don't give them anything that says "enhanced" or injected with broth or anything else. If it doesn't say that, then you can look at the sodium content and make sure it is LESS than 100 mg per serving. Most whole chickens will be about 80mg and some pieces I have found are up to 90%. Also, keep the "giblets" from the whole chickens to use later. The gizzards are considered meat as well as the heart but the heart & kidneys are organs.
That is a good place to start but there is more info out there and plenty of mentors that will help! We have @savemejeebus and @karenben here that know more than I do. I will also pm you some groups you probably should consider joining for more information.
As far as cuts of meats, I just buy what is cheapest or on sale. Manager's specials are good too. I don't buy them steak when stew meat or a beef roast will do. I also buy pork roast and the cheap pork ribs as well and cut them into meaty meals. Beef or pork heart is also considered meat and not organ and can be fed as such. Around here, I can get that cheaper than I can get a roast. A butcher is good to find for lots of reasons but mostly because they usually have meats such as bison, lamb, goat, etc. which you will eventually want to add into their diet. Variety is important!
At any rate, it sounds complicated but isn't! It is very easy once you get over the nervousness and jump in. Good luck and I hope you decide to feed your pup a species appropriate diet!