There's not really much to say beyond make room for and access to a battery or two. A little will depend on how you "go active" as some active setups may have pots fixed to a circuit board (most don't) and then you'd have to make sure you drill accordingly.
All that active means is that it is somehow powered by a battery (usually to boost or buffer signals) where passive is not. Most of my basses go together the same either way as my electronics cavity is large enough for one or two batteries usually. If not, I put in an additional cavity for the battery(s).
so if i want to build a active bass,I need to buy active pickups,and pre amp.
does the pre amp include all the wires and the pot switchs?
for example: http://www.guitarelectronics.com/product/HR45
Yep, that's a good preamp to buy. Different preamps come with different stuff so you probably should ask. It looks like that place sells the preamp with everything you need except knobs and a battery. Other places will sell just a preamp with the wiring coming out of the module and you supply the pots and jack as well.
It's worth mentioning that this preamp works with passive pickups but there are active pickups as well. These have an on board op-amp that requires a battery to run. The op-amp doesn't have any controls but brings the pickups down to a low-impedance signal. You can then do passive volume and tone or an active EQ system. Most EMG pickups are active.
I have used one of the Bart pre's, and it workes well. The only thing I did not like was that there is not a lot of room to move the pots beyond their prewired state, unless you want to solder longer wires in yourself. Easy process, though. I put the battery box in the cover on that bass.
