Ah, it takes some delicate play to win as Byz in AGCEEP. It starts you out a bit truer to how the Byzantine "Empire" really was at that point: Utterly pathetic.
But if you have the fantasy events enabled, then my advise should hopefully help here.
As soon as the game starts, you MUST go for Smyrna. DOW Ghazi for it. You may have to reload a few times here - everybody usually does, but you'll get them eventually, and Karaman in most of my games will just sit there and let you beat them. Take Smyrna from them, and vassalize Ghazi if you can for a wee bit of extra income.
Karaman or the Ottomans will usually annex them in a while though, so don't ally with them. After this, disband all but a small part of your army. You don't want a big force sapping your treasury right now. If you can spare it, send personal gifts to the Ottomans to keep your relations as high as you can.
When a few years pass and your BB settles (around 1424 - 25 hopefully.), attack Albania, assuming they do not have an alliance with the Ottomans. If you've played right up till now, and are a bit lucky, you should be able to raise a big enough army - about 20,000 hopefully, and defeat them. Annex em'.
From here its really tough. Many people do different things, but what I did in my AAR on this forums, back when it was game-focused, was go to war with Venice. Now the only reason I did this was because they had annexed Athens. If you can avoid a war with them I'd advise doing so.
Anyway, attack Athens when your BB is again low enough, and vassalize them - DO NOT ANNEX THEM. If I remember correctly, there is an AGCEEP event where Athens will submit to you, allowing you to diplo-annex them via event, with much less BB.
By now, Morea should have been your vassal for about 10 years. With Athens under your control, you can now annex them. You now have five provinces! If you want, you can grab Theodoros and make it six. Regardless, if the Ottomans haven't dowed you yet, count your blessings, blockade the straits, and kick some Turkish ass.
Those are my start up tips for a new AGCEEP player. Hope they help!