Its my first rc project. I want to build a 50cm*30cm hovercraft that is able to run on land and water.
How many watts of dc motor should I use to get good speed?
And which battery will be good for it and of how much voltage?

Looks like you will be on your own on this one   This is because any power system selection starts with the weight of the aircraft.  Or one has to start with a particular power system, and then ensure that the weight of the model suits the power system.  In short, you should start with some idea about the total weight of the model that you are building.

Now in the case of hovercrafts, there are further complications.  For aircrafts, there are rules of thumb like "100watts of thrust is needed for every pound of weight" (or something along those lines).  When it comes to hovercrafts, such thumb rules may not exist.  This is further complicated by the fact that there are multiple designs for these, with different characteristics and efficiencies.

Search the earlier posts of VC. Scratch Built Hovercraft.
Nice idea and design. That will really help you.

http://www.rcindia.org/self-designed-diy-and-college-projects/scratch-built-hovercraft


Thanks

Yes please contact VC bhaiya. He has built a successful hovercraft.

HI been there done that : take a look though not RC https://sites.google.com/site/sevclubbangalore/

this was done way back in 2001 - and could be easily converted to a RC craft-

RGDS
irfan