Just getting into the hobby. My interest in the beginning will be WWI postcards especially the flyers. I know Germany treated their flyers like national heroes and used the postcards almost like baseball cards here in the states. A couple of questions: 1) whats the best way to protect the postcards? Soft sleeves and top loaders? 2) what is the general size of WWI postcards? What size sleeves and top loaders should I use? 3) hints to smart collecting? 4) did other countries print a lot of flyer postcards like Germany did?
1) The best way is to keep them from too much exposure to air and bright light. The sleeves you mentioned are the way to do it. Look for stamp supply companies to have a selection as "cover sleeves" or somesuch. Poly sleeves work and are the cheapest method; I believe they are still only side loading. I'd prefer the harder ones that are top loading that come in two thicknesses, 5 mil (slighty less than two sheets of paper thick) and 10 mil, both equally sturdy in my opinion. I'd get a pack of each type to start to see what you like best.
2) General common size is about 5.5 x 3.5 inches like US postcards of the period. The above sellers will give you measurements for holders.
3) Have fun!
For your area of interest, you will have to be willing to take part in the hunt for material. This could include online sources as well as any local stamp and postcard shows. Don't be intimidated by some prices you may see as they will be all over the place. You will have to learn (rough) valuation with experience, including those of mint vs. used cards, condition levels and autographs (real, printed, faked and forged). Clearly, von Richtofen's and Eddie Rickenbacker's will be popular and expensive as well as having a chance of having a forged autograph (if present).
The same dealers noted above might have photos, stereoscope viewcards, postcards of unidentified fliers and just plain autographs. So you might be extending your interests into those areas as well.
4) France, for sure. African-American Eugene Bullard served in the French army and air force and appeared on postcards, I believe.
As well as the imdividual pockets that hy-brasil mentions, you can also store them in albums, which is my own preference. Here, the albums designed for cigarette cards will hold postcards in their two-section pages. Lots of manufacturers - from Lighthouse down - make larger albums that will accommodate four cards per side.
About 20 years ago I started a collection of postcards from a prolific U.S. publisher. I did not know much about this publisher, so I decided to set a spending limit of not more than $1.50 for any card during the first year or two until I got my feet wet. In the years which followed, I gradually upped my spending limit and now just go ahead and buy any card when I find one I have not found in 20 years of searching. I waded into the shallow end of the pool and went deeper as I learned to swim. No doubt flier cards start off quite a bit higher!