slow draining toilet/tubs

lately, everytime we turn on the shower, the toilet bubles. when we try to flush the toilet, it doesn't flush, it just drains slowly. if we don't use the shower - like overnight, the first flush in the morning- the toilet will flush normally for the one time - then it again doesn't flush - it just drains slowly. at time we'll hear air buble in our tub if the toilet flushes. we have a stand up shower and a separate tub - jacuzi in our bathroom. the shower seems to drain ok. it's just when the toilet drains slowly, the tub also drains slowly...

help!

thank you