Before collecting the data, did you know which group would be larger if
there was a difference? If so, then you can use tails=1. Otherwise
(the difference could have been in either direction) you use tails=2.
This is discussed in more detail in any introductory statistics book.
Help on TTEST seems quite clear on type, assuming that you have a
background comparable to having covered t-tests in an introductory
statistics course. The choice is based on how your experiment was
designed, and involves more nuances than are easily addressed in a
simple newsgroup reply.
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