What specifically do you want to know? Men and women who are not NPQ (in other words, are physically qualified for unrestricted line) must choose that (with one exception). That means, you must choose surface line, subs (currently males only), USMC, or aviation (pilot or NFO, USN OR USMC). Men can also choose SEALS and EOD. There are variations on these (e.g., nuc surface). The one exception is that men and women can select the medical school option -- but that is a complicated process available to at most ~25 students/year. A tiny handful (1-3 per yr on average) might be able to do inter-service transfer to the USA or USAF.
Folks who are NPQ select a restricted line community such as Intel, Aviation Maintenance Duty, etc. These billets are very limited and only available to those who aren't physically able to go unrestricted line.
The number of billets available in each warfare specialty varies a bit year to year and thus, the number today may or may not be what you are looking at 4 yrs or more from now. As a general rule, the number of USMC billets will increase -- currently about 25% of each class goes Marines and that may increase to 33% over time.
Your service selection is based on a combination of your desires, your order of merit, and the needs of the USN/USMC -- not necessarily in that order.
Typically, the higher your order of merit, the more likely you are to get your first choice. However, you could be "voluntold" to choose a certain service selection or that organization may not want you (e.g., SEALs).
