Its easy - you have two options . 1) click the buy points button - this is not expensive and you can buy points to buy planes with. 2) you see the top right when you play and also in the plane select screen? This is the credits/points you have to buy things with (it goes up as you progress through the game) when its high enough you can buy a plane.
U need to buy the full version and then only can u buy planes. If u have enough coins and since u have 25000 coins u can get some plane not the best ones like the albatros because they cost 75000.