we will be in sarawak about a week. we want to se kutching (is this a big city), semenggoh wildlife rehab centre, bako national park (is it possible to go there early in the morning from kutching and be there all day, and then go back), batang rejang (is there some turist office which organise 2 days trip to go there, and how much does it cost- we want to sleep only one night in the longhouses), which national park is nicer-bako or kubah? thank you!

Kuching (= Cat in Malay) is a fairly small city as Asian cities run. It is expanding quickly though. But all the interesting stuff is about a half km from the Main Bazaar along the Sarawak River. All the backpacker accomodations, several quite nice, are within this city core...as are most of the fancier hotels.
Semenggoh is close enough to town that you can take a local morning bus (@ 7:30 AM about 2 ringgits) and get there by the 9 AM orangutan feeding, and be back in town before lunch. You can stay and look at the botanical gardens if you want to make a full day of it (bring a lunch).
There is a Tourism Office right along the waterfront, in the old courthouse complex at the end of the Main Bazaar. Don't be fooled by the one run by the city businesses...this one is directly opposite the "Square Fort". It also is the booking office for the National Parks. Tha's where you's get all the information you need about buses and boats to Bako, and book accomodation if you want to stay the night. Many people do day trips, though. You can take a local bus out there early in the morning (best to leave about 8 AM to avoid the heat of the day) and do some short walks including the magrove plank-walk, perhaps seeing proboscis or silvered leaf monkeys, Wagler's Pit Vipers, Wild Pig...and definitely long-tailed macaques.
Bako is far better in terms of mammals...almost none to be seen in Kubah. But Kubah is great if you want absolute solitude. Bako always has loads of tourists, overnighters and day-trippers. Kubah is definitely self-service...no nearby restaurant or canteen, though...bring your own food to cook. Bako has a canteen thatv serves basic Malay-style food and offers cool soft drinks, tea, and (sometimes) beer.
Lot's of local tour agencies will arrange trips up the Rejang and longhouse stays. Cost varies...helps to shop around.