Your water change schedule depends on tank stocking/size of tank/amount of filtration.
A lot of us big fish keepers do multiple water changes per week. If you think about it, in the wild fish get 100% water change every few seconds. So it's our jobs as keepers to give them the best quality water possible.
I do what's called "fin level" water changes every third day. You take the water down to the height of the tallest fish, 80% or so of the water.
Things to get as a beginner fish keeper.
Liquid test kit (not strips)(this needs to be able to test for Ammonia, Nitrites, Nitrates and PH)
Api sells a master test kit with all of this.
Could also get a water hardness tester.
Some way to change water. Can go two ways a hand pump siphon into 5gal buckets. Or a "Python" a contraption that hooks up to your water faucet to drain and refill your tank.
Depending on your water source a declorinator
Things to get a bit later
A hospital tank
Petco/Petsmart have $1 per gallon sales on tanks. Get a 20gal, couple sponge filters (run them in your main tank to get them seasoned). So if you have an issue with a fish you can seperate if needed to treat.