Once I installed Debian 8 Jessie it did not have APT repository.

(It happened due to using cd-rom insallation image.)

So, let’s add repository manually.

1. where is repository list?

It is located in /etc/apt/source-list

$ sudo ls /etc/apt

2. Check the available mirror repository

You can check available Debian mirror repository list below link:

https://www.debian.org/mirror/list

Choose one what you prefer.

3. Add mirror repository on list

Using editor to edit source-list (I use nano)

$ sudo nano /etc/apt/sources.list

I addedd at the end of file below repository

deb http://ftp.jp.debian.org/debian/ jessie main contrib non-free
deb-src http://ftp.jp.debian.org/debian/ jessie main contrib non-free

4. Save and close. Now ready to use use mirror repository

For example,

$ sudo apt list

This command will show all available packages from current repository.

That’s all! Congraturation!