Raising warriors for a battle was based on a levy system, called the leidang.

Viking Age: Everyday life during the extraordinary era of the Norsemen, by Kirsten Wolf, explains in this system the country was divided up into districts. Each district was required to provide one ship of a standard design with 40 oars and the men to row it. Men would have to show up with the required standard set of armament and supplies for a trip.

Each district was divided up into small localities with each area required to provide one warrior, his equipment, and supplies.

Book says for defensive battle all of the levy would have been called out.

For an offensive effort only a portion of the levy was called out, leaving a significant portion of the force at home to defend the country.

Sea battles would involve lashing together the ships so there would be a solid platform to fight from. Volleys of arrows would start the battle followed by hand-to-and fighting when the boats made contact.

Offensive campaigns outside Scandinavia would have the ships spread out and then land simultaneously. Pillaging would take place with an attack on the defenders if there was any resistance.

Book has a good, short 22 page summary of attacks and conquest in Europe, the British Isles, Iceland, and Ireland.