To DDoS a Website/Server you need to have a botnet or a great number of computers (or Hardware which connects to the web like webcams, Toasters, ...) which you've exploited. Otherwise it makes no sense cause if you try to DDoS a Target with only your own Computer its the same like ping the Target.
Then you have to find out the IP of your target (thats the easy part).
Mainpart: Simultaneously launch your target with all the clients at the same time. If your target is a single Computer: make sure the port your going to DDoS is open!
If you have enough clients which ping (don't quote me here) or try access you target at the same time, a computer will be very slow and a website will go down for a while...the reason is the bandwidth.
But: i think it's not a good idea to start to lern hacking with DDoS attacks.
You actually dont need to udp flood to DoS. If its a webserver, you can bring it down, but keeping one of the back end processes busy. If its prone to SQL injeciton for example, send a WAITFOR DELAY command, and set it to wait 60 seconds, and do that every 60 seconds, or with multiple sessions, and voila, the site is now down. You could also just use stress testers to bring a network down, you dont need to build a botnet. Mind you, all of this stuff is illegal if its not on your own network/webserver unless you have written consent (like in a pentest).