1) Don't Expect The First One to be Great
I know we all have our pet projects that we dote on, but unless you have done a lot of writing before, your first novel will be an experiment in what NOT to do. As much as I look back on the years I spent writing Through Dangerous Skies with nostalgic pride, I know that, quite frankly, it wasn't that great. Anachronisms are everywhere and the conversations didn't feel very natural or authentic. It never sold as well as The Paratroopers and Spearhead of Invasion have, and it didn't deserve to. It was a test sample. I learned a lot in writing it and the painfully unfruitful attempt to get it published, and I was able to come out swinging on the next try. (Still hasn't made much money, but there's always a hope, right?)
2) Research! Research! Research!
Did I make myself clear? You can never do too much research. Historical fiction needs to have some grounding in real events, even if characters and incidents are fictionalized. Don't be afraid to buy books and read them multiple times and take notes. Also, depending on the period, technique, field craft, and procedures behind tasks and functions that a character would be doing might need validating. Also, research what the common person of that era would experience. A WWII GI would be more likely to take a train or bus back home than a commercial airline flight, with a Vietnam era grunt might be more likely to ride a Boeing 707 back to Miami to visit family. If at all possible, try to understand the meaning and usage of words and terms appropriate to the era and place. Period literature, and (if applicable) films or radio broadcasts are a great way to start.
3) Don't Be Discouraged
Keep working on it. If you are passionate about the era you are writing about, and you have good stories to tell, you will continue to hone your craft. Even once I eventually finish Till Victory Be Won, I will have more to work on. I find the 1940's to be so fascinating that I doubt I will run out of ideas, even challenging ideas, for many years to come. You may not be the one to see the fruit of your labor. Sometimes good books take decades to become uncovered. Lamplighter Ministries, for instance, has built its publishing on good, solid books that have survived their authors. Other now famous works commonly seen in classic literature courses, like Moby Dick, didn't become popular until after their author's death. If you have a good, uplifting story to tell, don't be afraid to put it down on paper or a typed document. Somebody might eventually read it and enjoy it for themselves.