It is mostly straight coast so anywhere has swell. It depends a bit on your confidence level, eg on bashing through shorebreak and what is convenient location-wise.
There are easy entry points at Port Beach or Leighton's in Fremantle but this area has a bit less swell, then more exposed as you go further north (loosely).
Next spot north is Swanbourne, just north of Grant street beach past the small headland - in between Leighton's and Swanbourne there is a few reef sections and it is probably best to avoid this whole stretch for launching unless you have a local with you.
Then city beach beside the northern groyne (don't go to the south one as this is usually flagged for swimming), which is a bit more exposed but the only real pick between swanbourne and city's.
Then its mostly exposed beach break through until Trigg point, there is a little spot at Bennion beach you can get out at, then North beach (south of the jetty), then Marmion beach (Troy ave entrance).
After that, the north side of Hillary's breakwater provides a really easy entrance and some flat water, and you can get out the back to some moderate sized swell.
My pick for solid waves is North beach, it is sheltered for a reasonably easy entry and you are straight into some solid swell on a perfect angle for upwind-downwind. If that looks like a hard entry, then Troy ave has a sheltered deep section that lets you get up and out through the break. If it all looks too hectic, then either Port or Hillary's.
Downwind run options depending on length and commitment levels:
Port Beach or Swanbourne to City Beach [common SUP run]
City Beach to Sorrento [northern folk common SUP/parawing run]
Bennion to Sorrento
Port to Sorrento (longest realistic run you can do).
A lot of wingers go City to Sorrento, if you get in early the entry is fine, and then Sorrento is much more sheltered so the exit is not too much of a drama once the wind picks up and the swell ramps in.
There are dedicated swimming areas on most perth beaches at regular intervals, with lifeguards present. These are marked by red and yellow flags at each end of the patrolled area (usually 50-100m wide) - STAY 200m OFFSHORE and don't launch/come ashore in these areas (in particular if you are finishing at city's or sorrento you will be coming in near these).