Peco have a large number of books that include plans designed to work with their set-track system and that includes the numbers to buy the relevant trackwork. Takes out a huge amount of the guess-work if you are new to this.
Just from my own experience: A couple of important things to consider before you start:
1) Reach - can you reach every point on the board from the edge? Obviously for 6ft wide you will need access all round, nothing is more annoying than not being able to reach if you want to add scenery or nudge a stalled train.
2) Consider what you want to do with the layout. From the description I guess it is more about running trains rather than, say shunting. If you want to add some challengers, try adding a little yard - there are some excellent track plans you could incorporate (
a good selection here) quite easily into a layout and that will provide some fun.
3) Do you have a particular era or location in mind? A track plan can severely limit a layout, so make sure you have decided on what you want to do before committing. A small single track branch with engine shed would never not really work as a modern layout for instance.
And my best tip - keep it simple. The more complex the plan, the less likely it will ever get finished. Certainly for a first layout, the best advice is to keep things simple and just enjoy the layout, then later start making more elaborate plans when you know your limits and abilities and simply what you enjoy doing.
