A short guide to point you in the right direction.
Guide for New Players
How to Get Started
Before you do anything, here is what you need to know, BTD5 has many maps which have their own interesting and sometimes difficult mechanics.
Beginner maps have (usually) no extra things to worry about, one entrance, one exit, some water for water towers and a track the bloons go along, bloons go from the start of the track to the exit.
You should (if you are confident) play hard straight away on the easiest map you can find, work your way along the beginner maps on hard until you have enough money, you should have 7500 or above, at this point you have a choice to make:
Buy double cash, this helps a lot in the fact that you have double the starting cash and double the cash from bloons popped.
Buy tower buildings, my personal favourites are: Ice and Bloonchipper.
Ice has a really busted power that lets it de-camo bloons, pair this with 3-X arctic wind and you’ll find every bloon is normal and can be seen by everything, this is great for beginners.
Bloonchipper has an interesting economical advantage, it can collect bananas for you, not only invalidating banana farmers, but letting you save precious monkey money for harder modes.
As a rule of thumb, left path is better late game than right area.
The power of both paths is equal (mostly) except that the right path is an ability that isn’t always active (used mostly for tricky rounds like 63), and the left path is constant, but in some ways weaker.
This (should) be enough to get started.