Make alliances and trades with the Scottish and the French from the start (I think you'll be at war against the French at the beginning, so if you want to trade with them which is highly recommended before a real war starts then try to convince them by sending your Princess for Diplomatic negotiations, but do not send her to marry a French Prince/King, that'd ruin everything because your Princess has the best charisma out of all of your starting regular Diplomats so keep her outside of any marriage), in fact you should train one or two extra Diplomats excluding the one you have when you begin, and send them all across the map and do the same with every Christian factions. If you make alliances with Islamic factions the Pope won't like it very much and other Christian factions will soon regard you as a potential threat. As soon as the Pope orders a Crusade, obey. As required train a half-full flag of troops and enter the Crusade, but you don't have to actually go and attack the target of the Crusade. Just entering it will suffice.
Anyways... where was I....
Ah yes, so trading, very important. Also at the start don't train any other troops, keep what you have, and instead build Farms (Land Clearance) in all cities/villages. Then make Grain Exchanges (lowest levels of a "market"), which are required if you are to trade with other factions. Then only once your Farms and Grain Exchanges are done you can consider training more troops (since your Farms will help with the food production and you won't eat everything for your troops leaving your regular populace starving to death). Basically you need to manage your resources first instead of your military.
If you play the vanilla game (without mods, but still patched that is) then I think that the first nation that will attack you will be the French, or the Scottish, but I think it'll be the French, so as soon as you notice that they (French or Scottish) have sent a Spy in any of your settlements then you know what it means, they are preparing to attack, but that doesn't mean they will do it the next turn, but stay vigilant, just position your troops closer to your settlements to discourage any assaulting attempts even by the Rebels. Another thing to consider keeping an eye for is the overall happiness of your population. To keep them happy at the start without costing too much you'll need to build Churches. Also train Priests and position one inside each of your controlled provinces (outside of the settlements) to keep the Christianity healthy and the Heretics away.
Hope it helps.