Often, one would have at least a rough idea of the track plan before building the benchwork, so that the benchwork could be optimized for the track plan. But not everybody does it that way, so I'm sure you'll be fine!
Do have some ideas about your goals for your railroad, or the time period, or location, or type of equipment you want to run? For example, some people just enjoy watching trains run round and round the layout, perhaps through a particular type of scenery. Others concentrate on operations: switching cars around to create trains, delivering loads to customers (industries) along the railroad, picking up empties, perhaps running a passenger train or two between towns on their layout.
Some people model a specific railroad in a specific time and place. Others run whatever equipment they like from any and all eras and don't try to model any specific railroad.
What are the "must have" items for your layout?
If you describe what you want to do with your railroad, what scale you want to use (HO, N, etc.), and the dimensions of your table, perhaps some forum members can suggest appropriate track plans.
- Jeff