How long would it take us to journey to an alien planet ?

Yeah sorry Star Trek beat you to it with the deflector dish: https://memory-alpha.fandom.com/wiki/Navigational_deflector But it is an issue that would need solving.  

Can you show me an episode of Star Trek where they used this deflector dish? And what kind of radiation was used to scan for debris, comets, asteroids and such?

I’m not a Trekkie but here you go fast forward to 3min 30s Not sure what you mean about the radiation.

copy/paste from google.  The Voyager spacecraft are traveling at speeds of over 35,000 miles per hour (mph), with Voyager 1 currently being the faster of the two, reaching speeds around 38,000 mph. 

Radar waves or 'radiation'. Sound waves are also a form of 'radiation', but very slow, using air molecules as carrier. Some kind of wave form that radiates outwards from its source, and bounces back to a receiver. My point: if you travel at light speed or beyond, you'll have to have some sort of detection system that is able to detect smaller objects. You may have set your vessel on a course to some star or planet, but you'll have to be prepared for much, much smaller objects crossing your path. At light speed or beyond that could be a problem.  

Yes the problem you have highlighted is very real.   I have no idea what sensors star trek ships use.

In scifi books slower than light spaceships sometimes have a mountain of ice as a meteorite shield. These spaceship do not avoid collisions - as that would be impractical. Either the ice endures smaller collisions, or it does not and the spaceship is destroyed. The plus point is that the ice may serve multiple purposes. The minus point is that it adds a huge amount of mass to the spaceship - and for that reason in my opinion it is impractical.

Assuming there's either no wormholes or humans don't know how to find or use them, the trip would take much longer than human lifespans. The astronauts would have to put themselves in suspended animation for many decades, like they did on the first episode of "Lost In Space".

good question how long would it take us well if the bus trip going on that one I would not mind giving it a go

Given the average speed of a bus, it would likely take longer than the entire lifespan of the universe to even reach the edge of our own galaxy.