Find out what your college's rules are for being readmitted. If it's only been a semester or two since you left, you may not have to complete a full application and be formally readmitted, but rather may have to receive approval from the dean of your college to enroll in classes again. Your academic adviser in your program or major's office can help you find the correct information.
Gather the materials you will need to begin the process of applying as a re-entry student if you need to be formally readmitted. Obtain an official copy of your transcript that shows the college work you have completed from the registrar at your school. Pick up a paper application from the admissions office or, if the application is online, find the correct website. Call the admissions office if you need help finding the application, and be sure that you are using the application for reentry students, not freshmen or transfer students.
Complete the application, being careful to follow the instructions. You will likely need to report your grades from college and perhaps high school and will need to provide your transcripts from all institutions that you have attended to verify those grades. You may also be asked to submit official SAT or ACT scores. You will also be asked to write at least one essay, or personal statement, describing why you left college and why you want to come back. Submit your application to the school by its deadline.
Look for scholarships, grants and loans if you need them. File the Federal Application for Student Aid to be considered for federal loans and grants. Contact the financial aid office at your school and ask if they need any further information from you to be considered for additional scholarships or grants.