Shortening your borders asap is the key according to me but before doing so you need to build enough troops and thus need a few years.

I usually start as follows :

- Round 1 : build watchtowers in all provinces. It gives you the Builder virtue and raise loyalty. Start building any troops available (spearmen in Burgundy, militia in Switzerland, Emissaries in Swabia). Try to conclude alliance with the French (Emissary in Swabia ) and the Polish (Princess in Brandenburg). With VI the offer is usually rejected by the Polish and accepted by the French.

- Round 2 : Raise taxes to very high in every provinces. Build forts in all provinces, spear-workshop in Switzerland and Swabia. Send reinforcement to Lorraine, Burgundy and Bohemia. Try to get an alliance with Hungary and Italy.

After a few turns, if you are Lucky, the Polish should attack the rebels in Moldavia, Prussia, Pomerania or that other province east (forgot the name). Time to seize Poland while their troops are divided and beat them piecemeal ... Hold Poland and they are dead because all their other provinces are so poor that they won't be able to produce anything else than peasants and woodsmen.

Once you have secured Poland (about 1105), you should have enough troops on the western and northern border to check any attempt by the French and Danes. Time to go south and seize those juicy Italian provinces. Though battles ahead since the Italians have better troops than you usual spearmen and militia. Mounted crossbowmen and, if you can afford them, mercenary cavalry are a must. Try to kill as many knights as possible to get rid of the entire faction, otherwise they will withdraw to Corsica and remain able to attack your provinces until you get a few boats.

After that it is usually time for a pause. Try to get a stalemate with the French until you have been able to replace your early units with feudal ones. Once it has been done you have plenty of options : Hungary (but it is usually useful to keep it between yourself and the Byzantines), east and the many rebels or west (French and English - Secure you border but difficult and the Pope will not appreciate ...).

Not a walkthrough of course, merely a few suggestions.