The most important thing is to know what you want and to turn your idea into a layout.
If you know what kind of locomotives and rolling stock you want to run, you can make better choices.
Once you do, there are some general rules of thumb-
Do Not Make Spaghetti One common pitfall in track planning is trying pack EVERYTHING into a given space. Yards, industries, stations, engine service areas, turntables and roundhouses, they all take up space, and require lots of track.
If you find yourself with all track and no scenery, you have a spaghetti bowl.
As tough as it might be, its often better to pick just one or two key features for a layout, rather than trying to fit all of them in. If you have a small space, this is especially important.
Also, adding all that track often adds complexity and potential problems.
Less is More!
Broader the better
Where possible, use the widest radius curves you can fit in a space.
Do not compromise on curve radius unless you have too.
This is especially important on your "mainline" tracks.
Don't make it complicated
Never make your track plan "difficult" to operate on purpose. The novelty of those "Time-saver" layouts gets old fast. Even a seemingly simple track plan can actually be challenging to operate properly, and so you don't need to build in frustration.
Also, avoid unnecessary use of "Special" track sections like "Double-Slip" turnouts, as these can create reliability issues as well.
Ultimately, a simpler track plan will make you happier.
Make sure your yards have space!
If you build a yard, make sure that it will be able to handle your traffic. Yards are for sorting and making up trains. If your yard is packed, you won't be able to do that.
Generally speaking, when all your trains are "in the yard", it should only be half full.
For example, if you have a single 6 car freight train on your layout, the freight yard should have at least a 12 car capacity. A run-around track is also useful here.
Do your research!
You would do well to read up on the literature available on the topic, there are many finer points to track planning, and the only way to learn is to read!
As always, if you have any questions about planning or constructing your layout, please feel free to email them to me and I will be pleased to assist you.