I must "zameldowac" myself and need an ID card?
Well, I wasn't born in Poland so when I claimed Polish citizenship (Dad's Polish), I first received a letter confirming my status. You obviously won't need this if your birth is registered in Poland.
I live in Lodz, so my next stage was to go to my local 'delegatura' (local council) and 'zameldować (register). I had to show proof of residence and the letter confirming my citizenship. Christ the biurwa in the delegatura was a bitch. Seriously. She sat there with a face like an Easter Island statue with an arse full of razor blades. Anyway, after much crying and pleading, the cow registered me and I could move to the next stage.
Which was to go to the local 'Urząd stanu cywilnego' (USC - Registry of Births Marriages and Deaths) to register my Polish birth (and marriage, because I'm married). They processed my personal details for my ID card and calculated my PESEL, because, as you know, it's based on date of birth, sex and a couple of other things. Whether you're a homosexual / communist etc.
Yhen it was a short wait before I could go back to the delegatura and collect my card.
You should find the procedure a lot easier since your birth details will be in Poland. I'm assuming you're not in Poland at the moment? I would suggest getting in contact with the USC where you were born and asking them for advice, since as far as I know, they are the people who assign PESELs. Their number will be on the internet somewhere