While you may already know which newspapers you want to print your announcement, it's best to do your research. You'll probably choose a local paper from your hometown and your spouse's (especially if you and your mate's families live in different cities). You might even consider a national newspaper or choose to publish an announcement in your grandparents' hometown papers.
Maybe you only want to go local -- or just national. Either way, once you've narrowed down your selections, get a list of requirements from the newspapers right away. You need to know the word limit, picture size, deadlines and fees.
Different newspapers also require various details to be included in their announcements, so be informed before you start writing! After doing your research, you may decide that some newspapers are too demanding or expensive and rule those out from your list of options.