You can find a lot of suggestions from different coaches and authors about how they go about finding a starting point. Keep in mind that it's always just a starting point.
Here's a very simple approach:
1- Figure an approximate # of calories you need to maintain your current weight. I like to use 16 x BW and 18x BW to get a caloric range
2- Set protein at about 1.25g -1.5g x BW
3- Calculate healthy fats at about 20% of your total daily cals. This can be done with foods, supplements (I live on Flameout Fish Oils), or both
4- Figure the rest of your calories as carbs. With this figure you can then go about placing them at specific feedings/times where you will get the most out of them (ie. before, during, after training)
Now this is a VERY basic approach, and of course it ignores a lot of individual variables. What I would consider once you've gotten this far though is to adjust what end of the caloric range you follow based on what you're doing on a specific day. If you're not training one day, follow a plan calling for 16 x BW calories and even lower total carbs that day. If it's a really hard training day (ie. leg day), follow the 18 x BW plan, and maybe even jack up carb levels a bit more than usual.
Everyone has their own approaches, but I think this gives a nice starting point without getting too confusing. Hope it helps.