Does your bike rust when u leave it outside?

If you leave it exposed to the elements for long periods then yes. Usually you see this on abandoned bikes that have been chained up somewhere for a couple months and left unclaimed.

You can probably find things to treat it with to make it more resistant to that though, and if you make a point of trying to put it somewhere covered and less exposed to the elements and use it regularly then that may help delay that.

Would probably suggest trying to use covered bike storage where possible if leaving it there for more than an hour or so. Note also another potential risk of locking it up outside where it is also fully exposed to the elements is that it may be fully exposed to potential cycle thieves as well...

Hmmm coz i live in a accomodation and i think it should be safe in here. Would a bike tarp help?

As in student accommodation? I would not chain your bike up outside a student accommodation. Cycle thieves deliberately target university premises and student accommodation because they know students tend to get new shiny things when they go to uni, but often aren't very good about making sure they are secure or securing them in a safe location. Most uni cities have rampant cycle theft which predominantly targets the student population. I'd suggest keeping it in your flat in that case. Unless it's a secure storage (e.g. bike cage type thing) I wouldn't risk it.

In terms of keeping it outside if it were in a more secure place (e.g. in the garden/bins area which is gated off from the street of a private residential property, or it is a secure cycle store area) where the main concern is weather issues the yes a tarp would probably be a good idea though