That's awesome! I have 4 turtles. 3 red eared sliders (trachemys scripta elegans) and 1 ouachita map turtle (graptemys ouachitensis). Maps are adorable turtles with lovely patterns and a unique shell.
I would definitely recommended getting him in the larger tank as soon as you can. If he's
4 inches or larger he should be in a 40g or larger. By the time he's full grown he will probably need a 100g. They get big but it takes a few years, so you have time.
As for filters, turtles are very messy. I would recommend getting filters that are for larger tanks.
Get a filter that can be used in a 100-150g tank for your 55g tank (I'm serious, it makes a big difference). I would also recommend a
separate feeding tank. This can be a small 10-20g tank or even a large tupperware container. It'll help keep the main tank cleaner for longer.
Lighting they need heating and UV light. You can buy a dual bulb or you can get two separately. Basking is very important.
As for cleaning; a lot of people use
bleach and as long as it's rinsed it doesn't do damage. However, a lot of people use
vinegar instead. It does a really good job at cleaning. Mix half vinegar and half hot water and buy an aquarium scrubber.
Aquarium vacuums are also really helpful.
You'll also want to brush your turtle.
This helps loosen scutes. Do this once a week. Twice monthly with just water, and twice monthly with a half water and half white vinegar mix (avoid getting it into your turtles eyes).
Use a SOFT bristle tooth brush and make circular motions on your turtles shell. Rinse off and put back in the water. Keeping your turtles shell healthy should be top priority. Brushing, basking and a healthy diet will do the trick.
Shell rot and retained scutes and pyramiding are all things you want to avoid.
Make sure your turtle gets a lot of vitamin A, B-12 and C.
If your turtle is a girl she will eventually have to lay eggs.
Even if there is no male to fertilize them, she will lay eggs. You can boil them and feed them back to her or you can feed them to your snakes.

She will need a nesting box with soil at this time.
As for diet---
Younger turtles like high protein diets and will crave meats more than veggies.
Worms, crickets, snails and guppies are all good choices. You can also use
dried shimp or meal worms as a treat. If they are given too much protein they will start to pyramid, so the sooner you get them liking their veggies the better.
Dandelion leafs, turnip leafs, red leaf lettuce and green leaf lettuce are some of the best choices for veggies for your turtle.
They can also eat
butter lettuce, kale, romaine lettuce, carrots and sweet potatoes. These are not as good for them as the above mentioned though. But good for variety. To make carrots easier to digest boil them first.
You can feed them
mango, cantaloupe, papaya, bananas, strawberries, peaches, watermelon, pears, plums and grapes (remove the skin). Because fruit is so high in sugar they should be a once in awhile treat.
Amazon swords, anacharis, duckweed, water fern, water hyacinth, water lettuce, waterlily and pond weed are all edible aquatic plants.
Pellets should not be a staple food, but should be fed every couple days. Get ones with a lot of vitamins.
Calcium is also EXTREMELY important for a healthy shell. Get a
cuttlefish bone and put it right in the tank. Do not get those turtle shaped supplements, they are not actually good for the turtles.
Cuttlebones are best. You can also get a calcium powdered supplement and put it on your turtles food before feeding.
Another awesome food for turtles are "jello shots"! No, not real jello or alcohol! It's a mixture of either store bought powder packed with vitamins and minerals that you mix with water to turn into a gelatin like substance (I buy a brand called savory stew) -- OR -- you can make homemade jello shots by;
Ingredients:
2 cups of pureed greens
2 tbs ground turtle pellets (reptomin, mazuri, etc.)
1 tbs calcium power OR ground cuttlebone
1 tbs flaked tuna (optional)
1 cup boiling water
1/2 cup cool water or cool tuna juice.
2 packets Knox Unflavored gelatin
directions:
in a bowl mix the first 4 ingredients on the list until well blended. add the two packets of Knox gelatin to the cool water and stir for 1 minute. let sit while the water boils. add the cool water with gelatin to the boiling water and stir until the gelatin has dissolved. Pour the gelatin mixture into the bowl of veggie mix. Stir until blended and portion into ice cube tray molds. finish by sprinkling turtle pellets on top of the mixture. Place in your fridge and allow them to set.
Once they have set, pull them out of the mold by using a tooth pick and cut them in half or fourths (depending on the size of your turtle). place pieces in rows on a cookie sheet and freeze. once frozen, portion into plastic containers.
Something else you want to watch out for is
respiratory infection. This is the number one killer of aquatic turtles. Keep him or her healthy with good food and the water warm and this should not happen.
Hope this helps!
I learned about my turtles via everything-turtles.com and Facebook groups dedicated to turtles. I strongly recommend them. I am also going to post some helpful posters from the website.