neeny

Start with an era and think about your requirements. It's okay to be interested in everything because that's what an undergraduate curriculum does: it gives you the buffet style of art history so you can learn a lot about many things. But, eventually, you are going to have to figure out what area appeals to you and your personality most. 
 
Are you most interested in western or non-western art? 
 
Are you interested in a "hot field" or a traditional one?
Latin American studies and Chinese (somewhat East Asian, but particularly Chinese) are currently the most advantageous, offer the most scholarship and best acceptance rates. However, you may have more of a background in Renaissance or Contemporary, which are always calling for scholars and seem to always have job postings. I think the smallest fields are the decorative arts and architectural history, so you may have less competition getting into those disciplines but in tandem, there are a lot less job offerings. 
 
Are you good with languages? 
If so, tackling ancient or medieval/Renaissance may be a more realistic option for you because you'll need Latin, French and/or German for either discipline, and depending on your specialization, you may need up to: ancient or current Greek, Italian, Spanish, Dutch, etc. Having a reading knowledge would be most beneficial for these disciplines and it will make you the most competitive. 
If not, most other areas (American or British studies, Modern/Contemporary) will probably only require English and maybe another language (MA) or two (PhD) will suffice.
 
MA or PhD? Art History/Art Administration/Museum Studies/Architectural History/Conservation?
If you aren't  sure what career opportunity you want to do, spend some time looking at programs that specialize in these fields and see what they mention as jobs. You don't need to be a professor or a museum curator (both require a PhD) to get a job in the art field, although they may, arguably, be the "sexiest" position (and possibly the most of a headache since they are the most competitive). You could work in preservation, any other area of a museum (registration, development, membership), or go into the auction/art market with a MA with an BA in Art history. A MA would also not require you to hyper focus on one area just now. However, if a PhD is your end goal, many PhD programs seem to prefer those with a MA already, although they does not mean you aren't eligible to be admitted to a PhD program. 
 
I know this is not quite what you are asking for, but really no one can make that decision for you. For me, I picked my field because I thought the art I was looking at was the most appealing/beautiful and I never get tired of seeing it. There are styles/periods of art that I absolutely have no taste for, so marking those off were easy. Also, having a conversation with my advisor and telling him my most favorable interests helped because he was just like "Why don't you just study XX." Oh. Well that makes sense!