It really all depends on your network and the services that need to be accessed within and outside of that network. What type of network is this? Business? Personal? How many users? What classes of users are on this network? What applications and services does each user class need?
The network is a business network. I wanted to know about the basic routing that needs to be done.
That is, if I hook up the firewall in between the gateway and the switch(as shown below), what kinda routing rules or IP rules would be required to be able to access the internet ?
[The other firewall settings, i.e. services to block certain traffic can be configured later]
Internet ---->Gateway ---> Firewall ----> Switch -----> Multiple computers
Regarding the other question about class of users, I do not know about that.
Number of users would vary between 10-25.
Well you say "gateway" but is it the gateway or the modem? If your firewall will be getting assigned a DHCP IP address from the modem then your shouldn't really need to do much at all in regards to routing or IP rules usually. I can't say I've ever used that firewall but most default to allow high security interfaces (internal) access TO lower security interfaces (external). If you want to allow traffic from the external to the internal then I'd think you'd need to define some rules there.
Regarding routing, assuming the dlink will be operating in a routing mode and not transparent (a mode it might not even have) the internal hosts will point to the firewall as their default gateway / default route. The firewall will point to your ISP's defined default gateway. You may need to configure the firewall to hand out DCHP with this information... or maybe you're doing that else where....