Let's say you get 200,000 bells for the bridge. The next day go to Town Hall and talk to Pelly who whatever there two names are. She will ask you about the bridge and where to place it. YOU can only place it where the two spots so pick wisely. Also, You don't have to get them 200,000 bells straight up. You can buy how much a day you want to. Also here is some information.
200,000 bells = Bridge
500,000 bells = Fountain
700,000 bells = Lighthouse or Windmill. You have to chose one so pick wisely. Also you can change it if you give them another 700,000 bells.
Yeah, after donating it takes about a week for whatever you're getting to build. Also, you can't donate, say, 1 million bells up front and expect to get multiple things at once, you have to donate for one thing at a time.