Use the after you have talked about it the first time. For example:
This is a cat. The cat is brown
Use the when the person you are talking to already knows what you are talking about. For example:
I am going to the bank.
There is someone knocking at the door. It must be the computer repairman. I called him to come help fix my computer.
Use the when you talk about general places in nature like the lake, the mountains, the ocean, the beach, etc. For example:
Let's go to the beach.
I love to camp in the mountains.
Use the when you are talking about something that there is only one of and everyone knows about it. The sky, the earth, the stars (as a group). For example:
How many people are there on the earth?
The sky is so beautiful today.