Making money from advertising online can work in different ways. If you have a business that you want to promote then advertising will enable you to make more money from the business as long as you attract potential customers to your website and they buy things form you. However, if you do not have your own business, you can still make money from advertising by setting up your own website.
Setting up a website is probably a lot easier than you might think. You can make a free blog site and will not even have to buy a domain name or hosting unless you really want to. You can also find places where it is really easy to understand how to design a site as it has step by step instructions. Examples of this are wordpress and blogger. It is important to have a lot of content on the site and to keep adding to it. This is both because people visiting the site will want something interesting to look at and new things when they return, but it will help your position in search engines. These days it is also important to think about social media and how you will promote it using that. Think about your favourite social media site first and set up a page on there to try to drive traffic to your site and then share details or articles you have posted and things like that to try to encourage visits.
Once the website is established and has quite a bit of content, you can start looking for advertisers. You might want to start by using a system like Google ads or similar. They run adverts on your site when you place some code on there which is linked to your Google account. Then when people click on those adverts you get credited with some money, the amount ill vary depending on how much the advertiser has paid for the advert. Once this amount has built up, you will be able to get a payment from Google.
Alternative you could approach companies directly and see if they want to run adverts on your site, perhaps in the form of sponsored posts, banner ads or sidebar links. You can decide how much you charge them, but you might want to start off cheaper to start with and then charge more as your site grows.
Another way to earn is to use affiliate links. This might involve you reviewing a certain product and providing a link to it, which you will get commission on if they buy the item. Amazon and Tradedoubler are examples of companies that offer this service.
It is well worth thinking about the different options and which you think will be the best. If you do decide to set up a website you will need to be prepared to publish new articles on it at least daily in order to keep people visiting. The articles will need to be interesting, different and fairly long.