Doubles is a game of angles. A doubles court is only marginally bigger then a singles court, therefore 2 people can cover a doubles court much easier then a singles player covering a singles court.
Angles are most easily created via a volleyer, therefore the volleyer needs to be encouraged to get every ball within range.
I could type out a page of doubles tactics but lets start with the basics
1. Play the middle : play thru the middle as often as possible during rallies
(a) it gives your opponents no angles
(b) it gives your partner better net covering ability
(c) serve middle to give your opponent a restricted area to return the ball therefore enabling your net partner to "poach"/"cross" with little risk of being passed
2. Play low (or high) : keep balls on volleyer's toes or over their heads
(a) if you get both opponents at the net follow rule 1. But keep the ball low to encourage upward defensive volleys, allowing easier passes or volleys
(b) as mentioned before two players can cover a court easier so passing shots are harder to come by, so lob the weaker player, preferrably using a cross court lob (greater safety) or toward the backhand side (weaker smash)
3. Play as team : its not two people playing singles on the same side of the net!
(a) Communicate, if you know what your partner is planning you can either set them up or cover for them
(b) get both to the net as often as possible, but if you can't then get both back, avoid one up one back as often as possible
4. Play the weaker player : yeah its obvious but has to be said
(a) you can't avoid the stronger player because they have to serve and return, but if you have both players in an equal position then obviously you gotta play the weaker player.
(b) Breaking serve, strong player serving = good serve / weaker volleyer, weak player serving = weaker serve / stronger volleyer. See which is the easier to break and throw the kitchen sink at their service game
5. Have fun - its just a game