If you haven't already, I recommend reading the Tips and Notes which contain a lot of useful information for beginners about German grammar:

https://www.duolingo.com/skill/de/Basics-1

i should say if you scoll down when your on a lesson and it should tell you the diffrence and when to use

The authoritative way of getting the right answer for whether a word is masculine or feminine is via the dictionary. That said, as others have pointed out, there are some shorthand ways of figuring it out that might help.

Good question. I struggled over this for a while. And the final answer really is There is no answer. However like some people have said there is an answer for SOME words but many are just random. However if you are able to learn weather something's Masculine or Feminine or Neuter. Then it tells you a lot about how to use the word in different sentences. I have a book that helps me remember the The that goes along with the word. But it's good to learn them because they tell you about how else to use the word. Eg Der and Das- ein. But Die-eine. when in sentences in which the word is Dative: Der and Das-dem. And Die-Der. It's a long road to find the variations of all the different words but I found that google translate is pretty good in the way of telling me what the goes with my sentence.

Cheers cluney.