There are several apps for both iPhone and android. That's the easy part. In order to get to your camera over the internet, you need either a static IP address for your router or what's called a proxy service. Your software will need that to find your cameras over the internet. Getting the static IP address can be a problem because some internet providers will only give a static IP if you are a commercial customer. The reason this address has to be static is otherwise the provider changes the address whenever your router goes off line. Like in a power outage. It will wake up with a different address and you won't be able to find it anymore from the outside world.
The proxy is a web site that monitors your IP address and can still find you if it changes. You in turn just have to connect to that web site. Sounds complicated but it's not. Most web cams come with a suggested web site that does this for a few bucks a month, or shows you how to use a service like DNS.com
If you want more cameras it is more efficient to learn how to run them using port forwarding on your router.
We should bump this to a new thread if you want to know more.