You will need a phone, tablet, or computer with the Google Play Store app (or other alternative).
First, you need to get the game. Go into the play store. Then search BTD6 and if it shows up results choose Bloons TD 6. When you are at the installing screen, press the button to purchase it and choose what way you would like to pay. Normally it costs $6 but sometimes it is on sale. Make sure it shows up on your device's app screen.

Next, start the game. Press the app's icon on your home screen. It will go into the game. Press start. Wait for it to load, then press the play button. Choose a map that shows up. Once you have chosen your map, choose a difficulty. Standard mode will show up. Other modes will also show up for that map when you complete it on standard. Press standard.

Finally, play a game. Some of the towers look like monkeys. At any time when you have enough money, you can drag a tower in your tower section and place it anywhere on the map, except for where the bloons come out. The goal of the game is to get to round 40 (easy difficulty), 60 (medium), or 80 (hard), without running out of lives. Pop all the bloons before they get to the end of the track. Now press play.

Press a tower to see what upgrades you can get for it. Press the i button to see what those upgrades do. Leads are grey, metal bloons that cannot be popped by sharp projectiles. Camos are bloons that can only be popped by ninja towers, or other towers with certain upgrades. You might want to put down a second tower before you press play. Press the double arrows to speed things up. Press the little gear to go into the pause menu. Choose auto round start if you want it to start rounds automatically. It is best if you buy a tower right away once you have enough money for it. Rounds will go faster if you put towers at the start of the track.

I think you will have fun playing it and unlocking all the different towers, upgrades, and modes. Do not forget to open your daily chest on the left-hand side of the home screen!