When applying for medical school, your ultimate specialty is of little consequence. Most doctors don't decide on their specialty until during their 3rd year of medical school. Medical schools all have similar requirements during 3rd year, usually including: internal medicine, pediatrics, obstretics / gynecology, surgery, and psychiatry. There is somewhat more variability in what is required by each med school for fields like neurology, anesthesiology, radiology, dermatology, and surgical subspecialties like orthopedics, otolaryngology, opthamology, etc.

So - basically, the fact that you know you want to pursue psychiatry doesn't make that much different for you at this point. You will need to know what residency programs you are applying to as you approach the end of your 3rd year.

Undergraduate GPA and MCAT scores are not directly important in getting into the residency program you want. However, GPA and MCAT scores did determine which medical school you got into. And better med schools will help you get into better residency programs. Grades in medical school are probably somewhat of a factor in getting accepted into residency programs, but I'm less familiar with that since my med school was pass/fail. Performance as a third year student and fourth year student is more important - you will want to get great letters of recommendation and you'll want a good Dean's letter showing good performance on your clinical rotations.

Different specialties are more or less competitive. The competitiveness of a particular field is always changing as market forces and the nature of the fields change. Some fields, though, have been very competitive for many years - orthopedics, dermatology, radiation oncology, otolaryngology, general surgery, and I'm sure a few others I'm not thinking of.