Dressing for a marathon comes down to the weather, and your preferences. If this is your first marathon, you might not have much experience with long distance running. Hopefully though, you have at least run some distance runs.

Know yourself, and experiment during your training runs. What works for one person certainly might not work for you. For instance, I have friends who run in nothing but tight-fitted leggings and t-shirts. I absolutely hate running in both of these clothes though. Tight-fitting leggings make me feel constricted, and tops that cover my shoulders make me feel like I can't swing my arms freely. I always prefer shorts and a razorback tank top, even if it's chilly.

Research the area you plan to run the marathon, and find out what the weather is typically like during the time of year your marathon is set for. If possible, try to simulate some of those runs during your training. So, for instance, the weather for the Boston Marathon can be unpredictable. There have been years that it's been hot and years that it's been driving rain, windy, and cold. Try to plan some of your runs to be during a rainy or chilly day. It's not ideal, but it won't be ideal come race day either! And you might find that you have some quirky tendencies. For me, that looks like running gloves. I run in a tank and shorts no matter what, but I HAVE to have on gloves unless the temperature is above 70 degrees. And because I know this, I can plan accordingly.

And when it comes down to fighting colder temperatures, the secret to properly dressing for a marathon is all about the layers. There's a term known to runners as "throw away clothes" which are really cheap clothes from a thrift store that you're okay with shedding just before the race at the start line or during the race and running the risk that you might not find them again.

Sources:
https://www.runnersworld.com/ask-coach-jenny/what-to-wear-for-cool-weather-marathons