Annual salary: £37,572 (Glassdoor)
What do I need? Computer, graphic design software
There are a fair few similarities between this and the web developer. In fact, if you have a web developer nearby who's weaker on the design side, perhaps you could team up and offer some kind of package.
So, back to graphic design. First, find your niche. This could be working with website interfaces, branding, advertising, website animations or in local magazines. From there, set your price, check out the competition and find where those gaps in customer demand are.
Like the app developer, looking at freelance job sites is a cracking starting point to get the first clients.
You'll also want to look at your pricing structure. Are you pricing clients by hour, by day, by project? The price you charge could be influenced by the aforementioned competition in your niche so be aware of what's going on. It's also wise to take your skillset and level of experience into account.