Editing in the laughter would really just depend on your editing software. Are you using Windows Movie Maker? If you have the laughter as an audio file, you should be able to just import it into the program or even just drag and drop it onto the timeline.
If you don't have the actual file, you can get a lot of sound effects from Youtube by using software to grab MP3's from videos. I use "Free Youtube to MP3 Converter" by DVDVideoSoft. It's free and it does a great job. Just be careful when downloading software like that, as there are a lot of scammers out there with these types of programs, and when you're installing it you've got to make sure you don't say yes to installing any unwanted stuff.
You should be able to find a lot of great Youtube videos that will teach you stuff about video editing.