Take a deep breath...I planned both of my weddings (widow from first,) a neighbor's and my daughter's in less than three months. There are a lot of online tools - checklists, etc. that can be helpful.
First, decide on size and venue. That will help with a lot of the other decisions. If she is going to have a commercial cake and flowers, those would be next on the list. Vendors need a little notice, especially if she is going cake tasting, etc..., and not just store bought.
Invitations should go in the mail three weeks before the date, however, it is a really good idea to get this date out to family and friends as soon as possible. Email is just fine for this purpose!
She will need to pick out her dress before you can do the others and she will need to have a little time for alterations if necessary. If you have a David's Bridal near, I highly recommend them. Prices are reasonable, always friendly and will work in a time crunch.
I used to plan weddings, so I have lots of ideas for arrangements, decorations, reception, etc... If you have specific questions, I'd be happy to help!! Just IM me any time...
Congratulations to your daughter and your new soldier son-in-law!!