What's the easiest way to make money online?  

You can either earn money through

Take Surveys Online
You can try to make money by taking survey online. However, it is not possible to make a living by filling up surveys and you will not be able to make a sustainable income. However, it is the simplest and the most straightforward way to make money online as it does not require any special skills and qualification.
Below you can find a list of legitimate paid survey sites you can sign up.
Pinecone Research
PaidViewPoint
Opinion Outpost
Swagbucks
iPoll
Vivatic
Global Test Market
MySurvey
iPanel
Ipsos
However, there are pros and cons making money through paid survey sites.

Pros:
Easy and straightforward
You do not need any special qualification
Free and you do not need to have special tools
You can participate and take surveys anywhere you want
All you need is a laptop/ smartphone and internet connection
Cons:
Small rewards from taking surveys. It is possible to make $5 per month if you work extremely hard
Easily get screened out in the middle of the survey
Lose all rewards after expiration duration
It does not allow you to make a sustainable income. You need to constantly participate in taking surveys every day.
Affiliate Marketing
Alternatively, you can become an affiliate marketer.

I graduated as an engineer and after years of working 9-to-5, I got fed up with the routine work and decided to do something else that could earn me an opportunity to spend quality time with my family and make extra cash.

I started as a blogger and expected to make some extra cash like anyone else. To my surprise, it turned out well where I made more money than what I could as an engineer.

My preferred way is to become an affiliate marketer because it allows me to make a lucrative and sustainable income through blogging. You can easily make 300 - 500 per day by becoming an affiliate marketer.

To make money from blogging (affiliate marketing), First, you need to find your niche. A niche means a target or a subject that you are going to write about. A niche can be your interest, a hobby or a skill. Any subject can be your niche, for example, people blog about collecting match boxes, bug photography, work-out program, or even daily political scandals and etc.

Remember one thing is when you are finding your niche, avoid being too broad with your selection. For example, do not blog about Work Out Tips and Motorbike at the same time. Both of them fall on different categories and they just can't simply be put together. You could do a post about your niche (e.g. camera, guitar, trekking) and include affiliate links to any paid ones (e.g. Amazon or ShareASale).

Create great content is not enough to make a lucrative income online today. You will have to make use of social media platform to promote your content and drive traffic to purchase items via your product reviews.