Get a copy of John Armstrong's book "Track Planning for Realistic Operation" (Kalmbach). It is a classic, and worth its weight in gold.
Read the NMRA standards to make sure you have adequate clearances and practical grades and track radius.
Avoid the common newbie mistake of trying to cram too much track into too small an area.
Decide whether you want to run with DC or DCC.
Length is more important than width. A yard with three sidings that each hold fifteen cars is better than a yard with ten sidings that each hold five cars.
Avoid anything wider than three feet — you won't be able to fix the derailment at the back of a 4' table, much less clean the track.
Make sure you can easily reach all turnouts for maintenance.
Take a good long look at your train room. Draw it out in scale, and decide where you can put shelves and peninsulas and maybe a helix if you want staging on another level. Consider where things like doors and windows and closets are, whether you want duckunders or a removable section or a blob to allow access to same. (A blob is the place where you have to stick the 4' circle to reverse the train.)
The Armstrong book has a very good description of the basic types of layout — dogbone, folded dogbone, pretzel, point-to-point, etc. It has a good deal more, too, so buy it! (And no, I don't work for Kalmbach.

I
do wish that I'd bought it before I built
my first layout. It would have saved me much grief and expense.)
A single-track main line with passing sidings is a lot more interesting and a lot more fun than a double-track main line, at least in my opinion. It is also more realistic in the vast majority of situations.
Don't rush to lay track. Spend as much time as you need planning the layout, until you are sure it will work for you. There are numerous computer programs available to help plan a layout.
There are also 1:1 templates that you can lay out on your table to see how it looks. Use these before you commit to laying track! (You can make them yourself.) Then cut a few pieces of string five feet long, eight feet long, ten feet long, and lay them on top of you templates. These are your trains. Drag them around and see how they interact. Are your passing sidings long enough? Can you get a ten-foot long train into your yard without fouling the main line? Are your runaround tracks where you need them? You will save yourself much grief if you can answer these questions before going to the trouble and expense of actually laying track. you don't want to have to tear it all up a month after it's finished, nor do you want to suffer with a clunker of a layout for any length of time.
Spend your time now wisely. Read, draw, research, and when you get tired of that, build your rolling stock and detail your locomotives. Choose an era. Choose a roadname, or make up your own. Know what you want to end up with before you get carried away nailing track to a sheet of plywood. If you
must run trains
now, build a switching layout that you can later incorporate into the Main Event.
So that's my 2¢. Maybe even 3¢. Model railroading is a lifetime hobby, so there is no need for haste. I suspect the average medium-sized layout takes ten or fifteen years to complete, from benchwork to the last tree or building. (Is there a "last"?) There is no need to get all the track down in the first three weeks.
And remember, this is a hobby. It's supposed to be fun, so if you find yourself getting frustrated, do someting else, whether it's building locomotives or making trees or reading a good book on Alcos or designing a block detection circuit. There are many, many different facets of the hobby to explore. And if someone says to you that this is inaccurate or you did that wrong, well, they may be right, but most of the time they're just trying to make themselves seem important. They're not — this is
your hobby, not theirs, and you are the only arbiter of what's right or wrong, or what's "good enough". I count rivets, but only until I run out of fingers.
Cheers
Scott Fraser
Calgary, Alberta