Market value for the average person
Search online for “[your job title] salary” and spend some time with the first few results, which are usually salary calculators and aggregators. I recommend PayScale.com, salary.com and glassdoor.com as good places to start, but the specific sites aren’t important since they could change over time. Some sites are geared toward specific industries, so you may get better data by focusing on sites that cater to your industry if there are any.
What are you looking for? Most sites will usually show a distribution-style description of salaries for a job. They’ll have a range—low to high—and they’ll identify the midpoint of that range. The midpoint is a good approximation of what the average person doing your job in your industry is paid.
The more data points you can gather, the better sense of the average you’ll get. You’ll find that these sites might vary quite a bit, so you’ll need to use some discretion when determining which ones are more accurate for your industry. Many of these sites also have a job description to go along with the salary estimates, so you may want to give more weight to the salary estimates for jobs whose descriptions look more like your actual job.
How you compare
Then you should consider whether you’re “average” or if you’re above or below average for your job in your industry. Maybe you have more experience than the average person, or you have a complementary skill that allows you to contribute more in your job than someone who doesn’t have that skill. For example, you may work in marketing for a clothing company, but you also have a background in fashion design. Those complementary skillsets may set you apart from your peers in marketing. If you bring more skills and experience to the table than the average person who does your job, you may be able to pursue a salary above the midpoint you found with your research.
Next, estimate the market value for your skillset and experience in your geographic region.