|Buying and Owning Stuff|
There's not a lot of stuff you need as a French Horn player. Your band director will likely want you to buy a mute and this is basically the only important accessory for Horn players. You will probably never use a mute before high school. Once you're in high school, you will need a mute for sure. Recalling that the French Horn is already difficult to play audibly, you should buy a mute accordingly.
Humes and Berg Stone Wall Mutes: Last resort. Don't buy a stone wall mute unless it's a matter of being in or out of band. The only time I used a stone wall that I could play loud enough to be heard was in a pit band for a musical and that was me blowing my guts out. Drilling a small hole in the bottom helps, too.
Metal Straight Mute: Good option. Expensive, but best for long term Horn ownership. Your snobbier cohorts will be less likely to mock your taste in mutes if you have a legit straight mute. Also, fun. A metal straight mute with the bottom removed will still sound muted but produce even more volume than usual.
Trombone Straight Mute: The best high school option. A Trombone straight mute will usually fit a French Horn, but you can add cork if needed. Once you get the fit right, you've got a mute sound with about 75% of the volume of not having one. As a bonus, they usually fit Mellophones and Flugelhorns and other crap you may want to own.
Transposing Mute: Don't bother. You have a hand. What does this do for you? You can make a very annoying sound with good volume, but it's annoying and you have to use mute fingerings (usually add 2).
Glass Bottle: Yes. With sufficient trombone mute cork or other stopper, you can build a mute out of a normal 20oz (or larger) glass bottle and it sounds good. It can take a little effort to line up and get the intonation good, but it's a lot like using a Trombone mute, just cheaper and more fragile. On the bright side, Trombone players are less likely to steal your mute.
Practice Mute: I've never used one, but I guess it might be handy if you live in an apartment.