For me the first sign was itchy flakey scalp, which for ages I thought was just dandruff, but eventually it got stronger and I scratched a few really big bits off that made me worried.
I also had a few spots on my legs that took a while to heal. I kept picking at the scabs and even when there was no redness or blood bits left it would keep getting really thick skin covering over. With Psoriases the skin plaques can appear at random on your skin or in some cases it can occur after a wound, piercing or tattoo.
It's also quite common for the skin on your elbows to get ticker and drier too. a lot of people have this and don't realise its Psoriasis. it doesn't always spread and some people only get very mild amounts.