There are services that can be enabled/disabled using the GUI (Like the startup application) and then other ways of doing the same through the terminal.

For the Terminal you have several options. Open a terminal (Type "terminal" in the dash for example and open it) and keep on reading:

Enabling / Disabling a service

To toggle a service from starting or stopping permanently you would need to:

echo manual | sudo tee /etc/init/SERVICE.override


where the stanza manual will stop Upstart from automatically loading the service on next boot. Any service with the .override ending will take precedence over the original service file. You will only be able to start the service manually afterwards. If you do not want this then simply delete the .override. For example:

echo manual | sudo tee /etc/init/mysql.override


Will put the MySQL service into manual mode. If you do not want this, afterwards you can simply do

sudo rm /etc/init/mysql.override


and Reboot for the service to start automatically again. Of course to enable a service, the most common way is by installing it. If you install Apache, Nginx, MySQL or others, they automatically start upon finishing installation and will start every time the computer boots. Disabling as mentioned above will make state of the service manual.

Temporary enabling/disabling services

To stop and start services temporarily (Does not enable / disable them for future boots), you can type service SERVICE_NAME. For example:


sudo service apache2 stop (Will STOP the Apache service until Reboot or until you start it again). 
sudo service apache2 start (Will START the Apache service assuming it was stopped before.).
service apache2 status (Will tell you the STATUS of the service, if it is either enabled/running of disabled/NOT running.). 
sudo service apache2 restart (Will RESTART the service. This is most commonly used when you have changed, a config file. For example in this case, if you have changed either a PHP configuration or a Apache configuration. Restart will save you from having to stop/start with 2 command lines)
service apache2 (In this case, since you did not mention the ACTION to execute for the service, so it will show you all options available for that specific service.)


This of course can vary depending on the service, for example, with MySQL it would only mention that it is missing a parameter. For other services like networking service it would mention the small list of all options available.

UPSTART

If we would wanted to use the official Upstart way (Note that, for the moment, not all services have been converted to Upstart), we could use the following commands:

status SERVICE - This will tell us if a converted service is running or not. Note that this is deprecated in favor of start, stop, status &amp; restart. It will also tell us if a service was not yet been converted to upstart:

A converted service would typically output the current status (Starting, Running, Stopping...) and process ID. A non converted service would give an error about an unknown job.

Some shortcuts may only work with the service command above but not with the commands below unless they are 100% converted to upstart services:


START - sudo start mysql
STOP - sudo stop mysql
RESTART - sudo restart mysql
STATUS - sudo status smbd


SYSTEMD

Starting with Ubuntu 15.04, Upstart will be deprecated in favor of Systemd. With Systemd to manage the services we can do the following:

systemctl start SERVICE.service - Use it to start a service. Does not persist after reboot   

systemctl stop SERVICE.service - Use it to stop a service. Does not persist after reboot

systemctl restart SERVICE.service - Use it to restart a service

systemctl reload SERVICE.service - If the service supports it, it will reload the config files related to it without interrupting any process that is using the service.

systemctl status SERVICE.service - Shows the status of a serviceTells whether a service is currently running.

systemctl enable SERVICE.service - Turns the service on, on the next reboot or on the next start event. It persists after reboot.

systemctl disable SERVICE.service - Turns the service off on the next reboot or on the next stop event. It persists after reboot.

systemctl is-enabled SERVICE.service - Check if a service is currently configured to start or not on the next reboot.

systemctl is-active SERVICE.service - Check if a service is currently active.

systemctl show SERVICE.service - Show all the information about the service.

How to enable or disable services?
I am a mathematics Ph.D. student who are in my final stage of completing my Ph.D. programme. My advisor is visiting an institute for a few months. The institute that he is visiting is in another country; because of the visa requirements and many other practical reasons, I cannot go there with him.

Now my progress is, I have all the main results ready (most of them have been written down) and I need to finish writing my thesis in about three weeks. My advisor has guided my through all the mathematical difficulties; now he left me behind to write the thesis all by my own. Furthermore, he has clearly indicated that he will NOT help me with the writing process. 

Now I need to figure out many things all by myself, from LaTeX to the organization, from the usage of languages to drawing pictures, etc. Also I need to verify the correctness and validity of all the results; he mentioned that he will not carefully read my draft. And I do not have much time to finish everything. I am feeling somewhat stressed and overwhelming. 

In this situation, should I seek assistance from other professors/lecturers who do research in a similar field, or should I try to figure everything out by myself? Should I hire someone to proofread my thesis when the draft is finished? 
