My wife have been cleaning an apartment twice a month for the past year. She was not given a W2. She made over $3000. How does this income be reported to IRS?

That is self employment income.   You can just type Schedule C into the search bar at the top of your return and it should give you a JUMP TO link to go directly to it.

To add a business or Schedule C….
Go to Business tab-Continue
Business Income and Expenses
Profit or Loss from Business, click Start or Update.  

You use your own name, address and ssn or business name and EIN if you have one.   You should say you use the Cash Accounting Method and all income is At Risk.   

After it asks if you received any 1099Misc it will ask if you had any income not reported on a 1099Misc. You should be keeping your own records.  Just go through the interview and answer the questions.   Then you will enter your expenses.

You need to report all your income even if you don't get a 1099Misc. You use your own records.  You are considered self employed and have to fill out a schedule C for business income.   You use your own name, address and ssn or business name and EIN if you have one.   You should say you use the Cash Accounting Method and all income is At Risk.   

After it asks if you received any 1099Misc it will ask if you had any income not reported on a 1099Misc. You should be keeping your own records.  Just go through the interview and answer the questions.   Then you will enter your expenses.

Self Employment tax (Scheduled SE) is automatically generated if a person has $400 or more of net profit from self-employment.  You pay 15.3% SE tax on 92.35% of your Net Profit greater than $400.  The 15.3% self employed SE Tax is to pay both the employer part and employee part of Social Security and Medicare.  So you get social security credit for it when you retire.  You do get to take off the 50% ER portion of the SE tax as an adjustment on Schedule 1 line 27.  The SE tax is already included in your tax due or reduced your refund.  It is on the Schedule 4 line 57.  The SE tax is in addition to your regular income tax on the net profit.

Here is some IRS reading material……

IRS information on Self Employment
http://www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/Self-Employed-Individuals-Tax-Center

Pulication 334, Tax Guide for Small Business
http://www.irs.gov/pub/irs-pdf/p334.pdf

Publication 535 Business Expenses
http://www.irs.gov/pub/irs-pdf/p535.pdf