How can I see background process in Ubuntu? And kill unnecessary processes?

From the terminal, ps -ef will list all the processes. See man ps. See man kill, man 2 kill, man killall, man nice, man pkill, man renice, man 7 signal, and man skill to mess with processes. However, simply killing a process that you think is useless may be a mistake. The system might restart the process, or something you depend on might depend on the process you killed. Learn what the processes do, and look at /etc/init/ and /etc/init.d, /etc/rc?.d, man service to see how processes are started by the system.

Using GUI, you can use System Monitor

Or from terminal you can use

ps aux | less

To view every process:

ps -A or ps -e

All processes running by a user:

ps -u username

To kill a process, either find the process name and type:

kill -9 processname

or kill the process ID (PID):

kill pid

Stop/suspend a process:

ctrl-z

Source:^{Man Page}

There is also the tool "htop". It is like "top", but has lots of other capabilities.

In a terminal enter:

sudo apt install htop

My main tool here is top

type top at the command line in a terminal window

You'll get a list of the process that are running, listed by cpu usage. Wait a few seconds for it to gather more stats before proceeding.

This is my main tool in unix for killing runaway or unwanted processes. They are likely to be near the top of the list. Note their pid and press q and then either 15 (soft kill) or 9 (hard kill).

Here you see me killing a Chrome process:

The process should go away. Then type q to quit out of top.

If you find you are always killing the same processes you can also use kill or killall at the command line, for instance if top has shown several java programs taking up cpu you can quit out of top and do killall java
kill and killall use 15 (SIGTERM) by default but you can override this with killall -9 [process] or killall -s SIGKILL [process]

Another good tool to use will be Glances.