Your mentors are correct in that most physics programs only take a relatively small number of PhD students each yer. That is because they make a commitment to funding them throughout their studies. In this case, funding means a stipend as @Pentaprism mentioned plus full tuition paid.
Yes, it is important to have research experience for a strong application to a PhD program but you have had some over the past summer and if you can get involved a bit more during the academic year (if your family obligations permit) then that should be sufficient.
Regarding grades, yes a high GPA and a very strong GRE is needed to get into the most selective programs but there are plenty of programs that you can likely get into which are very solid. I hate this idea of a “good school”. There are big programs and small programs, highly selective programs and less selective programs, “good” is a much more difficult concept. Many students in physics have this idea that the only PhD program worth attending is one of the 10 most selective. This is just not the case and if you limit yourself to applying only to those, then you are selling yourself short. In graduate school, it is probably more important who your thesis advisor is than the name of the university on your diploma. It is that advisor who helps you get that first job and after that it is up to you to make your career. I have been a physics professor for over 30 years at Illinois Tech and we are certainly not listed among the “top” physics programs in the country mainly because we are small compared to the most well known programs. However, my PhD students have never had a problem getting a job whether it is in industry, national lab or university. I can say the same for most of my colleagues. If you have a GPA above 3.4 or so you will be a good candidate for many PhD programs. Just make sure to apply to a few dream programs and at least one you would be happy attending which you are pretty sure would admit you. When choosing the programs you are applying to, make sure that they cover the areas of physics you find most interesting by looking up the faculty and seeing what they publish.
I would say that most graduate programs will take into account that you studied at UCB. This kind of undergraduate program requires you to take a full range of upper division courses and is an excellent preparation for a graduate program.
Finally, the American Physical Society has something called the [url="<a href=“http://www.apsbridgeprogram.org/%22%5DBridge%5B/url”>http://www.apsbridgeprogram.org/"]Bridge[/url] program which you would be absolutely eligible for as a first generation college student and member of an underrepresented group (women) in physics. This will get your name in front of many good programs if you have not been admitted to the programs you apply to. Honestly, if you are starting out with a 3.8 and finish with an overall GPA that is 3.4 or so, plenty of programs will be admitting you.
Bottom line, don’t stress out so much. Do your best with your courses and try to get as much research experience as possible so that you can get strong letters of recommendation. In a year when you start applying for PhD programs, make sure that you apply to a range of programs which have the kind of research that appeals to you.
Good Luck!