new to a used 2008 yaris please help!

[SezyVisa]

thank you for all the help, this whole week, i spent a lot time reading many posts here and tried to learn stuff about my car so i can do whatever i can with the car by myself. so far here are what i got done:
+ Replace engine oil and oil filter: i used mobil 1 high mileage synthetic oil
+ Replace cabin air filter: bought from napa: ultra 8
+ Replace engine air filter: bought from napa: ultra 8
+ Inspect tires, wear and adjust pressure: new tires
+ Engine coolant: add a bit more coolant : $20 toyota pink coolant
+ Inspect brakes, brake lines, brake hoses: still nice brakes have like 80% left
+ Inspect brake fluid- still full--- when do i need to change the brake fluid????

Here are few things that i probably need to get done soon
-Change serp belt: i check mine, the belt seems having very thin line crack every 1/4 inches, when should i change this? i read on forum on some post said i could buy one from amazon and get jiffylube to change it for $20

-Replace engine coolant: i went to toyota and bought their coolant, was onsale for $20, read the direction on the bottle how to flush, seem easy, probably i can do it myself. how many quarts do i need for the whole replace engine coolant process?

-Replace Automatic Transmission fluid: i'm still looking into this see if i can do it myself, tips and advices on this part???how many quarts Toyota Genuine ATF WS do i need for this whole process? should i use the toyota brand or i can just use any brand name? If i cant do it myself, should i find another auto shop to do at cheaper price instead of dealer?

Thank you