Hi Benny
I would only fish the Harbour as a last resort, you will catch lots of small chequers anywhere in the harbour, the odd plaice up the wytch channel. Try around Brownsea Castle for pollock, wrasse and pout. Ragworm in the harbour is top bait, sandeel and crab close second.
As Mike says above the Poole outer patch would be a good place to start. This time of year you should have no problem catching Bream, wrasse, pout, Pollock, smoothies, occasionally Bass and the ever present lsd's, to name but a few. You won't need a lot of weight here (2-8oz) so fishing light will give the best sport. 12lb class tackle will be more than adequate copeing with most situations and you can fish lighter at times. All the mainstream baits will work here, crab for smoothies and other species, ragworm will attract all but usually smaller species, Mackerel will catch all especially lsd's,squid for the Bream, sandeel or fillets of good for most species
Hope that helps
Martin