You first need to know if it is a cc3d flight controller using "libre pilot or open pilot" or a naze32 flight controller using "betaflight or cleanflight."
The two are not inter changeable. If it is not the one you have it will not open a serial port to connect to the quad. Check the web site you bought it from, it should be listed as that model is shipped with both depending on where you got it. You need to determine which you have because setting up or altering the flight modes is done in one of those programs. Your transmitter is probably already set up with a flight mode switch, but regardless, you can change it if you know what is loaded on the flight controller.
Also check the website you purchased it from, chances are there will be a manual that explains all of that.
When you get that figured out, buy a half dozen sets of dal 5040 tri-blade props, a half dozen antennas, cheap ones, and a couple sets of new legs. Then buy a soldering iron with a fine tip, solder, flux, a lighted magnifying glass on a stand, a small set of screwdrivers and 2mm hex bit. All of which you will become very familiar with in the near future. Small wire cutters, wire stripper, and a cheap heat gun, a small glue gun and an assortment of heat shrink tubing will also come in handy.
Batteries, if it is not the pro model with 20 amp esc's, 3s 1300 to 1500 40c will work fine. Anything larger will weigh it down too much making it sluggish. If it is the pro model get 4s 1300 with 75c.
One thing they do not tell you when you buy your first mini quad, it WILL spend more time on the work bench than in the air. The learning curve is HUGE. Welcome to quadcopters.
BTW, i still have my eachine 250 racer. have upgraded the motors, esc's, flight controller, camera and i still fly it. It is very stable in the air. Great little quad once you stop crashing it so much. My only complaint is having to desolder the esc's to replace an arm. On the bright side, you will get really good at soldering in a very short time.