Definitely do more research on your turtles and don't just impulse buy them. They're going to be pets for many years so it's your obligation to take good care of them. If you're thinking of small red eared sliders, that tank will be okay for a while, but males and females get to 8-12 inches respectively. You generally want 10 gallons per length of shell... so when one turtle gets to 8 inches, they need 80 gallons - and if you have 2 x 8 inch turtles, that needs 160 gallons. Turtles are a big investment but I enjoyed them for a decade. They are messy eaters but if you can feed them in a separate tank (and let them do their business in there) it'll make cleaning a lot easier.
As for keeping fish and turtles together... baby turtles might not be able to catch/eat the fish, but adults will. To me at least it's kind of silly to keep both together. At least if you care about the fish you're getting, you wouldn't house it with its predator. This is a fish forum so it would be like keeping a goldfish with a huge oscar and hoping the goldfish survived. It's food for the oscar and it's food for the turtle. Yea they can live together but turtles alone are a big enough deal I wouldn't attempt to tackle 2 pets you have no idea about.
So to sum it up...
#1 Stop, slow down. Do not rush and buy anything living yet.
#2 Do your research on the turtles you want to buy. As far as I know, no turtle is going to stay that small at 1/2 inch. Those are babies and will become large fast. If they're red eared sliders like I mentioned, they need more than just water - they need land and special lighting. What kind of lighting? Do your research online and get a book.
#3 Don't attempt fish and turtles. You're going to struggle with both if you know nothing about either going in. Like others said, turtles need warm water while goldfish need cold water; the 2 just don't mix if you want both alive.
#4 If you want both alive, you're being naive and again, do more research on both potential pets before jumping in. There's no pressure here that you need to fill that 55 gallon tank up today; it's not going anywhere so hold off until you know more.
#5 Make sure you're serious in owning a pet for many years. Don't let ignorance fool you. Yea the turtles are cute - but they grow up and require larger tanks, varied diet, etc. You wouldn't keep a cat in a small box for its whole life, so don't keep a turtle in a tank too small. Others might do so but you will know better after doing research so please be responsible and don't contribute to animal abuse.