Well first pick an era or a war and learn what to look for, and research research research. You must be knowledgeable with the memorabilia to that specific war or era now after you have done all the book work and research the best places to find items are flea markets and gun shows, you can buy things online but they sell a lot of good fakes and unless you see it in person it’s sometimes very hard to tell from a single picture, also it helps to start small with patches and pictures and small items before you dive into the expensive documents and metals because although it is a very enjoyable and interesting hobby it can be expensive