Hi Phailure,
This sounds like a really fun long-term project -- but I second Barrett's suggestion that you find a mentor. This is a complicated project and detailed advice is beyond the scope of this forum; it also will inevitably involve lots of electronics work, which is notoriously difficult to troubleshoot over the internet. It would also be good to have someone around who's familiar with all the safety concerns. In addition to things like sawing and soldering you'd be working with high voltages, and materials that are continually bombarded by particles of high enough energy (walls of the device, etc) can actually become radioactive...
... which raises the question: To what energy do you want to accelerate particles? A cathode ray tube (
http://en.wikipedia.org/wiki/Cathode_ray_tube), used in old monitors and TVs, is the simplest form of particle accelerator. As a starter project, you could build one or buy one and turn it into an oscilloscope:
http://www.instructables.com/id/How-To- ... illoscope/
http://www.youtube.com/results?search_q ... cilloscope
To get to energies at the current frontiers of particle-physics research requires accelerators that are physically enormous, requiring years to build and funding from national governments, even multiple countries (see CERN, the LHC, Fermilab, etc). As a result many of the results are derived from the combined efforts of
hundreds of people. There are some people who work on building components (especially detectors), some who write programs to interpret the data, some who spend all their time figuring out what theories the data is consistent with. This might be of interest:
http://www-d0.fnal.gov/public/index.html
So hopefully the above gives you some perspective for the purpose of contacting a potential mentor, probably a university professor or staff scientist at an accelerator. On that note, there are also accelerators that are used not for particle physics research but just to produce known high-energy particles for materials characterization and other experiments; some locations in the US are the Advanced Photon Source at Argonne National Lab (Chicago), the Spallation Neutron Source at Oak Ridge National Lab (near Knoxville), and the NCNR at NIST (Maryland). Fermilab, also near Chicago, is probably the biggest high-energy particle physics center in the US. However, there are people at many university physics departments involved in particle physics, including those geographically distant from where the research actually takes place due to the large-scale collaborative nature of the field. Anyway, you could approach someone local who does the kind of work you're interested in and ask if you could help out around the lab in some way or even if they'd help you with a project for a science fair. If you don't live near a university, try putting "summer research high school" into a search engine -- some universities have internship-type programs for high-school students. You'd still probably want to contact the scientists you'd want to work with.
Good luck,
Amanda