There are several ways to go about adding work experience to your resume. I think the best place to start is to outline the specific sections you should/would like to add to your resume. To an extent, this will be dependent on how you are going to use your resume. For example, when applying to graduate school, educational background and research experience might be more important than work experience, and thus should be sections listed first. Work experiences and leadership experiences (club participation, volunteer work, etc.) might be more vital when applying to a job. Further, you might decide that you want to divide your work experience up to be more marketable to a specific job, e.g., making a section that is specific to work you've done in marketing when applying to a marketing role.
Your resume may also depend on what other activities/interests/skills/experiences/etc. you wish you provide on your resume. I'd recommend checking out a few examples online to get a sense of what options are available. A quick Google search of resume examples should do the trick. Generally, you include your name, contact information, and a brief statement of who you are and what you'd like to accomplish, your career goals/what you are looking for in a next opportunity. A section on education is usually present, inclusive of University Name, GPA (if strong), degree earned, any awards/recognitions received. For work experience specifically, Scott is right about wanting to list the company name, dates of employment (month and year is usually sufficient), and a description of your work. I'd add the title of your position/role as well. As you add further work experiences, it is good to list your most recent experience first and continue in reverse chronological order.
When writing the description of your work, write a bullet point for each major activity/project you accomplished and skill you developed. Try to be specific about accomplishments, any quantitative or qualitative evidence you may have for success, e.g., Designed and implemented a marketing campaign that increased daily website viewership by an average of 10,000 a day. Attempt to use action words when writing these bullet points, e.g., Initiated a program that..., Designed a research study in order to..., Created..., Managed..., etc. Be sure to use key words that are specific to your role/field. Marketing jargon, while maybe not understood by everyone, will certainly be understood by recruiters and hiring managers within the marketing field.
In addition to writing a description of your work, it is also important to add any skills that you developed in your role to a separate section of your resume solely dedicated to the skills you possess. Any software programs you've worked with, languages you know, research methodologies you are able to implement, etc.
I'll reiterate that viewing examples of resumes is one of the best places to start. Beyond your standard resume template examples, it's really good to review resumes of people who are in your field. Checking out LinkedIn profiles of individuals who've had similar roles to yours or have roles you'd like is a good place to get inspiration. Reviewing job descriptions of roles like yours or roles you want is a good way to find key words that might be good to add to your resume. I'd also recommend getting in touch with your student services/career services department at your school to see if they can help you refine your resume further.