Step 1:Create a simple topology like this,
Step 2:Click on PC 1->Config,assign Gateway in this case,10.0.0.1
Step 3:Click on Fastethernet and assign ip address and subnetmask,
In this case ip 10.0.0.10 and subnet mask 255.0.0.0,Now close PC 1 window.Do like this for PC2 with appropriate ip and subnet
mask.
Step 4:Now Click on Router R1,then click on CLI(Commad Line Interface).
You will see like this,"Continue with configuration dialog? [yes/no]:".Give "no" and Press enter.Now you will go to user mode,
Step 5:now give "enable" and press enter.Now you get into the Privileged Mode,now type "configure terminal" and press enter to
get into global configuration mode.
Step 6:Now configure router interface with ip address and subnet mask then give no shutdown to make this interface and line
protocol up(i.e. Carefully configure ip address with proper interfaces in this case f0/0 and f1/0,f is short form of fastethernet.
Router(config)#interface fastEthernet 0/0
Router(config-if)#ip address 10.0.0.1 255.0.0.0
Router(config-if)#no shutdown
Router(config-if)#exit
Interface Line protocol on FastEthernet0/0, changed state to up
Router(config)#interface fastethernet 1/0
Router(config-if)#ip address 20.0.0.1 255.0.0.0
Router(config-if)#no shutdown
Router(config-if)#exit
Interface Line protocol on FastEthernet1/0, changed state to up
Step 7:Now lights on all ports become green from red.Now click on PC1->Desktop->Command Prompt.
Step 8:Now give this command "ping 20.0.0.10" and press enter.you will get,
connectivity between 10.0.0.10 and 20.0.0.10 is ok.Now PC1 communicates with PC2
Another way of checking connectivity is,select "simple PDU packet" from right side of packet tracer and select source PC and
Destination PC.You will get response at right bottom of the pacter tracer window.