How to capture DHCP packets in wireshark

Well, your assignment alone leaves room for interpretation.

As Ethernet (at least up to GBit-Ethernet) is defined by it use of CSMA/CD, it might be enough to capture just Ethernet frames.

As YLearn noted you wont see any collisions in a switched network (unless something is terribly broken). Even in an environment using Hubs (good luck finding some) or a shared Media like good old 10Base-T you may not see these packets. These packets are most likely filtered out by your network interface. To capture them you have to used specialized Network interfaces.

What you should see is a counter for collision, e.g. with ifconfig on Linux or show interface on Cisco IOS

Your scenario is somewhat confuse. If I uderstand well you want to filtered DHCP request on L2.

You can achieve it by using the function DHCP Snooping features available on your platform.

You could set all ports in untrusted mode so only DHCP reply from the switch will be accepted.

DHCP Snooping - configuration guide

DHCP snooping acts like a firewall between untrusted hosts and DHCP servers. You use DHCP snooping to differentiate between untrusted interfaces connected to the end user and trusted interfaces connected to the DHCP server or another switch.

Which is what you are looking, filtering DHCP communication between your hosts and upstream DHCP.