I have not shown any income for the past seven years, and receive about $900/month in Social Security. This year I will earn about $30,000. Will this affect my SS income, and if so by how much (approximately)?
It depends. It's complicated: if you are under your Full Retirement Age (FRA), then benefits are reduced by $1 for every $2 earned over $17,640. In the year you reach FRA, they're reduced by $1 for every $3 earned. And if you're over FRA, they're not reduced at all. At least if I have correctly interpreted the SS web page: https://www.ssa.gov/planners/retire/whileworking.html
PS: As Dilip Sarwate notes in a comment, your $30K earnings this year will almost certainly (given that your current benefit is only $900) increase your benefit in future years. The benefit is calculated using your highest 30/35 (don't remember exactly which) earnings years, indexed for inflation. So the $30K will replace your lowest earning year, giving a higher benefit amount.
(If I've understood the SS material, something similar is also the case if benefits are reduced due to earnings: the reduction factors into an increased benefit after FRA.)