We can already book space flights into low orbits (provided we are very rich indeed), but what about going further? Lets build an interstellar space craft. We will need to think about the following:
The nearest star is Proxima Centauri which is 4.3 light years away (25.28 million million miles away). The current fastest space ship would take about 15,000 years to get there. However, because there is very little matter in space, if we can make bigger better engines we can go much quicker.
Science tells us we cannot go at the speed of light, so lets aim for a quarter light speed. Even now the voyage will take 20 years or so, but this is within the realms of possibility in a few decades from now.
Alternatively we could look at bending space and traveling down wormholes. This is currently science fiction and so has to sadly be discarded at present. However, if you were the scientist who works out how to do it, you could probably demand a seat on the flight if you wanted it.
So our crew are going to spend 40 years together (there and back) so to ensure that one doesn't go insane and kill everyone else, there will need to be a good number of people. We know that small villages of 20 people survive entire generations, but below this number only families seem to survive without outside input. So our space ship will need either a family (like lost in space) or a crew of at least 20 people.
We don't currently have the facility to put humans into suspended animation (like in the film Alien) and NASA are not currently funding any research in human hibernation, so our crew will have to be awake for much of the journey. We have make massive leaps in virtual reality so keeping people entertained shouldn't be a problem. However, the latest 3d gaming console is going to get really stale after a decade or two. Then there is the issue of what games are suitable when you are on a long distance space flight.
Keeping fit will be a lot harder in space than it is here on earth. Without gravity human bodies do funny things. Firstly there is Space Adaptation Sickness which usually only lasts a few days but is something space shuttle astronauts had to suffer. It is basically a bad form of seasickness.
More serious is the longer term effect on our muscles. In the micro gravity environment of our space ship there is no gravity for the crews muscles to work against and after just a few days a deterioration in general muscle tone can be detected. Over a long time, the muscles will shrink and lose their strength, leading to twitching and a loss of fine motor control.
After just 211 days in space on the Mir space station, cosmonauts Berezovoy and Lebedev were so weak they had to be carried out of their spacecraft, and were not able to walk for a week. What would our crew be like after 20 years?
A solution is to mimic gravity by using constant acceleration. This is a common image in space movies, because it makes sense and seems feasible (and means they don't have to bother attempting to film weightlessness).You do it by rotating a giant wheel (like a hamster wheel but super-human sized) and the spin creates the feeling of gravity.
The film 'silent running' features a biosphere in space and lots of research is being conducted into growing fresh food in a space environment. It is unlikely that there will be many live animals for food on board but recently laboratory grown meat has appeared and this may well be an option (although for the sake of their health the crew will probably be sticking to a fairly vegetarian diet).
Beyond the Earth's protective magnetic field, space is fizzing with dangerous radiation. Without protection our crew would be sick within weeks and dead within months. Alongside the background fizz of cosmic rays, radiation sickness from a solar flare could kill an unprotected astronaut within days if they were exposed to it.
So our space craft will need some sort of shielding and possibly extra shielding for periods of solar storms.
With all this space for the astronauts, a biosphere, pods for artificial gravity, stores of food, huge engines and masses of fuel, it will be huge. Think about an ocean liner in space.
Of course we will need a lot of power to achieve a speed anywhere near a quarter light speed. Ion thrusters are a new form of engine being researched by NASA which may provide a solution. These thrusters are very efficient at turning tiny amounts of matter into acceleration.
If we get to Proxima and find aliens we will look a bit embarrassed if the ship looks rubbish, so I suspect we will get a designer to give it a decent paint job.