the SAT I is different from the SAT II (subject tests).
all universities and colleges require either the SAT I or the ACT. those two are interchangeable. you do not have to take both, but you can. then you can choose to send only your ACT scores and/or your SAT I scores, depending on which test you did better on.
SAT II (subject tests) are supplements. this means that while the SAT I (or ACT) is always required when you apply for colleges, SAT II (subject tests) are tests that focus on one specific subject (such as US. history, French, Literature, etc) that count as way less than your SAT I scores, but many schools (usually more prestigious ones) require you to take subject tests.
for example, if you live in california and want to attend a UC (for example, UCLA) you have to take the SATI (or ACT) and you must also take two SAT II (subject tests). But for a csu, state university, only the SATI (or ACT) is required.
the SAT I is always made of three parts: Critical reading, Math, and Writing. you cannot separate them, and must take the entire test in one sitting-- resulting in one total score each time you take the test.
each section, critical reading, math, writing, is 800 points. since there are 3 sections, it totals to 2400.
subject tests are also individually out of 800 points, but like i mentioned earlier, are supplements to the main test, which is the SAT I (or ACT). they are also taken on a different sitting/ date.
your best bet is to wait until after or around your sophomore or junior year in high school. see what subjects you excell at. if you're really good at US history or Spanish, buy the SAT II subject test: spanish (or whatever subject) study book, and take the test for that subject. these will be totally different from an SAT I study book or ACT study book.
good luck!