you have much to learn young one.
1 st - use sprites to create fields
2nd - since you want pvz game i assume your enemies walk a straight line, put a sprite on that field and check it's X and Y. spawn enemies on that X and Y, if you have more then 1 line, then random Y between 2-3-4-5 Ys (depending how many lines you got)
3rd - you will have to do some logic of hp / stuff and more.
4th - when you destroy last instance you removed that asset from layout, that's why it's prefered to put one instance outside layout (just drag and drop it outside layout (zoom out first) )
5th - make some global vars where you calc your hp/points/whatever and use them for your game
i can't help you much more because you didn't provide anything specific to solve, but you will get the just of it by working. a lot of working.