How do I get a Dynamic IP?

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There are two dynamic IPs involved here. One is your externally visible IP. That's the one you're worried about. That is given out by your ISP, either dynamically, or statically. Typically, most ISPs use a dynamic IP. Static IPs usually will cost you extra. Look at the configuration of your router's WAN connection. If it is using DHCP to get its external IP, then it's dynamic. Static IP requires you to enter a specific IP, plus the netmask and DNS nameservers.

That other IP is your internal (private) IP on your home network. That one you really don't have to be concerned about since it's not visible externally, i.e. outside you home network. It can be either static or dynamic.

I hope you understand that an ISP-issued dynamic IP is quite stable. It won't change unless you do something drastic like rebooting your router (and maybe not even then). That's not very conducive to warding off DDoS attacks. If you were thinking that your external IP changed regularlyduring a session, it doesn't.