I advise you use google before posting a thread next time, but
1.) ip flooding is highly illegal, Not sure why but it destroys their computers sometimes :\
2.) Theres many out there, I won't provide a link but search on google, theres a way to ip flood with only command prompt by the way so you might want to search that up :y:
3.) They catch you by simply tracing your IP, From what I know say I ip flood you, you report me to the police or your ISP, your ISP(Internet Service Provider) gets the ip of who has been flooding you and gives to the police, the police then track down the IP to a location and then they have you basically. You may even be caught if under VPN/Proxy sometimes.