Ah... how to forecast... tricky...
I use windguru a lot, so I'll stick to that for now.
First of all, look at the swell - the three measurements are size, period and direction.
Size is pretty obvious, but is hugely affected by period - that is the 'gaps between waves'. The bigger the number the more powerful the swell. Depends where you are exactly for this - in the UK for instance, you need a 11s+ period for it to be a decent wave, in sydney anything over 8s is surfable. If its 5s, say, then its just windchop and not really a wave. If its 7-8s, then it'll be a wave but messy.
Then direction - will the wave hit directly, or glance off the beach you're at? Local knowledge is key here.
Then wind - strength and direction is everything. Ideally you want a light breeze (<10kmh) blowing offshore (ie out to sea). If its a howling gale blowing at the beach, you'll just get that white water.
So an ideal situation would be a reasonable sized swell, a large period, the swell hitting the beach, the winds light and offshore (also remember tides).
So read the forecast and pick where and when you need to be.
Takes a lot of practise so do it daily whether you're surfing or not - where would you surf? And then check the surf reports to see if you were right.