What's the best way to learn a new language?

I'm learning spanish and when I learn something new, I forget what I learned the last month. For example, I was learning directions last month and then I'm learning family this month. I forget what I've learned last month because I'm learning something new. And also when I learn a phrase e.g. "estos son nuestros los archivadores" I forget to say "son" or sometimes I say something like "estos es nuestros" or estos son nuestras". I muddle plural with singular or I muddle masculine phrases or verbs with feminine phrases or verbs