Here's a pictorial guide to making a quality tuna fish sandwich:
Start with a can of tuna.
Open the can. 
Leave a little bit of the top still connected to the side, that will make draining the tuna easier. 
Make sure you drain the tuna as much as you can, that will reduce the "fishy" flavor that tuna can sometimes have. 
Add the tune and a dollop of mayonnaise and mix. I prefer to add the mayo first, so that way you can use the same fork to extract the tune from the can. 
Make sure you mix the tuna up thoroughly. If you bought chunk tuna, like I do, it will be easier to mix. Many people like to buy whole tuna, because it is less fishy tasting. I prefer chunk tuna because it is a lot less expensive, and I've heard rumors it has less mercury in it. If you drain it properly, add enough mayo, and include a little pickle, you won't notice the fishy flavor. (I actually like it - anchovies - YUM!)
The mighty pickle! Carefully slice it up and put it no the sandwich. 
The finished sandwich. Thing of beauty, ain't it? Serve with some potato chips or pretzels for a tasty, healthy, and inexpensive lunch.
