One of the most important parts of your layout is the track. You will need to decide track or battery power, and if track power, the type of system you will use.

The type of power and control is closely connected to your type of track, rail, and turnouts.

My advice is resist buying transformers, track, etc. in large quantities until you have made this decision. You really need to decide a number of things first. Track will be your largest single investment, and the least fun to change later.

First question - track vs. battery power:

If you are a newcomer to the hobby, you really need to decide this. It's an important decision. Battery is not always the cheapest and easiest way to go, although the battery people are much more vocal about it. (everyone starts off with track power, and many battery people are converts because they had trouble with track power. Track power people started in track power and never got disgruntled)

If you know your answer, then read the Track and Rail Material section. This might help you decide on what rail material is appropriate for your use.

I'm using Aristo stainless steel track. I use DCC and track power.

Sectional or flex track:

The best idea is to use flex track, you can make nice gradual curves to any requirement, and you have fewer rail joiners. Unfortunately, it's more difficult for a newcomer to lay this out. In my case, I have a very constrained area for the trains, and have made the curves as broad as possible. I use sectional track and a track planning program. This works very well and allowed me to get bigger curves and more track in a smaller area. 

I've already got track or decided, but I need help on getting things to run right:

If you have derailments or have never looked at Track standards, I'd strongly suggest the Track Standards section. Following these standards, and also gauging your wheels properly will make the most significant improvement in the operation of your layout (after leveling track).

I still have problems. 

Turnouts are where over 90% of derailments occur. I have specific sections on Aristo turnouts, both from my own work, and Ted Doskaris' "vignettes", check the menu on the left.

Notes on radius and diameter:

Most people from smaller scales will naturally express themselves in radius.

Well, LGB, who basically "started" Large Scale, used Diameter, for what reason, people can only speculate. (Probably to know what space a simple loop would occupy, since space is ALWAYS an issue)

LGB also expressed it's Diameters as "R" curves (could it be more confusing?"