You'll need to purchase an inexpensive
home router w/ several (usually 4) 'ports'.

The DSL modem plugs into the router and
the PC & Mac also plug into the router.
Yes, both the Mac & PC can be on-line
simultaneously.

The router takes care of all the problems
such as DHCP address assignment, NAT
address translation, firewalling etc.

In the US, they are about $60-100 USD
depending on features.

I use an Asante FR3004 but the Linksys
or SMC or any one of a million others
works.

See 'practicallynetworked.com' and learn
more.