if you want to fit bigger wheels to your car you need to check three main thing so they will fit your car propperly.
1) PCD this is basically how many studs/bolts hold your wheel on and how far they are apart from each other ie 4x100
2) OFFSET this is a bit more complicated ,if you imagen a line down the centre of your wheel where the wheel fits to the hub,then offset is a measurment of how far offcentre this line is compaired to the rim of the wheel ie ET 49
3) ROLLING RADIUS this is the measurement from the centre of the wheel to the outside of the tyre.this will affect how far one turn of the wheel will travel along the ground (circumfrance of a circle type of thing).the bigger the wheel the lower the profile will need to be to keep the rolling radius the same.the numbers on a tyre will help you calculate this. ie 215/35/18.
215 is the width of the tyre across the tread
35 is the profile (this is not a measurement in mm it is a percentage of the tyre width )
18 is obviously the size of the wheel
ok dont worry about calculating the size yourself have a look at this link for a tyre calculator and a better discription of the above
http://revolution.alloysuk.com/
tried to keep this as simple as possable for you hope it helps