the notion of gender in French is one of the most difficult points to grasp in the language, and especially for the English-speaking world.
French distinguishes the masculine from the feminine whether for people as for "objects". there is also the "neutral" gender (who is not male nor female).
below 2 very light description links, but which can be used as a basis.
"le vs la" http://www.frenchtutorial.com/en/learn-french/basics/le_la_les
in english with french sample
know that in French for example: "une armoire" (a cupboard) on one side and on the other hand "un meuble" (a furniture) on the other, yet a cupboard is a piece of furniture ;) (une armoire est un meuble)
other sample .. "un arbre est dans la forêt" (a tree is in the forest) we say "la forêt" (a designed forest) and une "forêt" (one of forest)
But, "un arbre" (a tree) can't say "le arbre" !
-> "les arbres" or "des arbres" (trees) is ok (
there are no grammatical global rules (except for certain cases), and it will simply be necessary to know if it is an "le/un" or "la/une".
this is one of the mistakes (funny for the French) that all non-French speaking foreigners do the most
You learn it n time
Here a review on the knowledge of "gender" at school (9 year olds)
practice, and get corrected ;)