As there are several bus companies going from Phnom Penh to Sihanoukville, you'll have the opportunity to choose freely from one of these. They pass Kampong Speu, so you just have to book a ticket to the provincial capital or you go a bit further depending on where you would like to go too. The most buses are departing from the Southwest corner of the Phsar Thmey (central market). The price shouldn't be higher than 6000-8000 Riel per ride. The buses depart almost every hour of the day, especially in the morning and midday (no buses after 4pm). Online Bus ticket can be purchased in advance.
Most Share Taxis are leaving Phnom Penh in the morning hours, as there is still business to make, when they approach in Kampong Speu. They leave also from the Charles de Gaulle (Southwest from the central market). The prices are around US 3-4.
Now very easy, you can book Private Taxi before your arrival through online.
Motorcycle Touring Info
Coming with the motorbike from Phnom Penh to Kampong Speu, you'll probably be taking the national highway No 4 in direction of Sihanoukville. As the provincial capital, also named Kampong Speu isn't that far away (approx. 48km) from Phnom Penh it's a one-hour ride with the motorbike. The street is in best condition, but the traffic, especially coming close to Phnom Penh might sometimes take your last nerve.