Standard Mode: You have to set Proxy Server address in Browsers. Technical: Browser will request your Proxy "Hey please catch for me Google.de"
Transparent mode: No Setup on Client neccessary, as soon as a http/https traffic outgoing hits your Sophos, it will get intercepted and filtered without notice [;)]
The address would be the address of the interface that the client system is connected behind (internal?). By default, the port is 8080, but can be changed in WebAdmin. There's a number of KB articles about the web filter that you may wish to review.
Thats of course a possibility. But please Keep in mind: For transparent mode to be functional you Need to set the IP of the Interface the transparent Proxy is set to as Default Gateway. (!)
Transparent means: As soons as a packet destined to Internet is recognized, it will be passed through the transparent Proxy Service of Sophos UTM BUT Internet traffic has to hit this Interface/IP you set the transparent mode to.
So in other words: To have transparent mode working you will Need to set Default Gateway to its Interface IP (via DHCP e.g)
In fact, you can use both Transparent and Standard on the same interface. I normally configure a Transparent Proxy as the default. Then, I configure a Profile in Standard. If the client is configured to use a proxy on port 8080, the Standard Profile handles it, if not, the default Transparent applies for port 80.