Listing all the challenges related to transporting humans safely to another planet could fill up a book or three. But one of the toughest problems to solve has everything to do with weight. The heavier a spacecraft is, the more fuel it needs to escape the Earth's gravity.
A journey to another planet would last for several months. Assuming you're either going to set up shop on a new planet or plan a return trip, you'll need plenty of supplies to keep you alive. Those supplies have weight and volume, requiring more fuel to get you up in space in the first place.
One potential solution to this problem is to build a space elevator. Here's how it works: We put something with a lot of mass in geosynchronous orbit around the Earth -- that means it will stay in orbit above a fixed point on the planet's surface. Then we attach a cable between the orbiting mass and an anchoring point on Earth. Now all we have to do is build an elevator that can climb the cable out into space!
It sounds like science fiction, but many engineers and scientists are working on building space elevators. Compared to launching a rocket into space, a space elevator is a bargain. The elevator could take equipment and even humans into space. Once there, we could assemble spaceship pieces and build a craft in space itself. There's no need to launch the craft from Earth because it will already be in orbit.