(I've answered this question offline enough times that I thought it would
be worthwhile to include it here.  The FAQ is distributed widely enough
that people may happen upon it through non-Usenet channels.)

 In order to access *sci.astro*, you need an internet service provider
(ISP).  This could be a large commercial provider, like AOL or Prodigy in
the U.S., or a more local one (check your phonebook under "Computer
Networks" or "Internet").  If you're enrolled at a college or university in
the U.S.  (or overseas?), talk to your computer center; many colleges and
universities are now providing free Internet access to students.  If you
don't have an ISP, you'll have to choose one.  If you're interested in
reading the *sci.astro* groups, as you search for an ISP, you'll want to
ask the various contenders if they provide access to Usenet and
specifically to the sci.  hierarchy.  If they don't, or can't tell you,
that's a bad sign.

 If you already have an ISP, you'll have to read their documentation or
talk to their tech help.  Some ISPs provide Usenet access through a Web
browser (like Mosaic, Netscape, or Internet Explorer), others provide
access through a dedicated news reading program like tin, rn, or GNUS.
There are many different possibilities.