Spaceships have specially designed bathroom for astronauts use. Toilets have various restrains (foot loops, thigh restraints, etc.) so that astronauts will not float away when he/she is using it. Also, due to weightlessness, the toilets rely on air and vacuum pump which creates suction to remove the waste. When urinating, astronauts use a large tube that is connected to the bottom front of the toilet. This tube also has air circulating through it carrying the urine to a holding tank. If you think this is too mich of a hassle, just wear diapers instead. no, this is not sarcasm, it works.

In the absence of gravity the collection and retention of liquid and solid waste is directed by use of air flow. Since the air used to direct the waste is returned to the cabin, it is filtered beforehand to control odor and cleanse bacteria. More modern systems expose solid waste to vacuum to kill bacteria, which prevents odor problems and kills pathogens.
Defecating is not easy in space station and spacecrafts. Because the diameter of feces collector is only 10 cm, astronauts must practice a lot to make sure that they can tie their ass very closely to the collector(don't laugh, i'm trying very hard NOT to laugh too, at least make an effort)Any leak would cause the feces fly everywhere, and that would be an embarrassing disaster. if it happens then it'll be POO WARS! in space, not something i like to do in my spare time...-_-)

OR..... you could....

get a bag and put its mouth on on your anus(not IN yr anus please). take some duct tape and make sure you get a good seal. then you pee or poo, when you are done, dispose of it IMMEDIATELY!

Note: DO NOT take the bag and use it as a bomd,its a messy business and you don't want to be the one cleaning up