Hi Veronica. Tuna is a pretty hardy fish. The common mistake with tuna is over cooking it. The key is to try to get fresh sushi grade tuna. Most natural/organic stores with a seafood department have it, or you can order it online but you will pay for it. Many cooking methods work, but I prefer grilling or searing. Get the pan screaming hot and add a small amount of oil. Pat the tuna dry and add to the pan. Let it sear until you see the proteins coagulate to about 1/4 of the tuna, flip and repeat on the other side.
Tuna can be marinated but really takes on the flavor of marinade so I would not do it but a couple of hours. I make my own sauces that I learned through chef Todd Mohrs web cooking classes. Most of all have fun with it.