The short answer is, it depends. Each site and preceptor is going to be different. Some preceptors are great and will allow you to advance in autonomy, while always being close enough for help when needed. Some preceptors are control freaks who will never allow you to see a patient on your own, no matter how simple the reason for the visit.
At a good site you can expect:
Review of the schedule prior to beginning patient care. Who is on the "guest list" so to speak, what problems are they coming in for today, which ones are likely to be student-appropriate (depending on your level of experience at a given point), and a tentative idea of which pts you will see during the day with support from the preceptor. Having said all that, in primary care the schedule is only a starting point and very quickly can go straight to H@ll as soon as they phones start ringing at 0800.
Once you have an idea of which pts you may see, you can poke around in their charts, see what meds they are taking, problems list, etc. At the beginning you may go in with your preceptor and see the patient together; he or she should observe your interactions with patients, exam skills, comfort level, etc. prior to sending you into rooms by yourself.
Once you're ready for more independence, you may go in and introduce yourself to the pt and ask if they will allow you to see them and then your preceptor will see them and approve your care as well. You will come out of the room after history & exam and speak with your preceptor. At this point you will give report on the patient, your exam findings, and (in my case) 3 differentials. Then describe what tests or further exam techniques or history questions may be needed to eliminate 2 of the differentials and hone in on the most likely diagnosis. Then describe what treatment you would recommend (meds, physical therapy, OTC comfort care, call 911, etc.) and wait for your preceptor to make suggestions, share observations, give further direction, etc.
Granted, it does not always work that way. But it would be nice if it did!