Your new here a an excited to get started
 Slow down take a deep breath an look around

See how things are done when it comes to posting no need to post the same thing in two spots

 From what he is saying  all it needs is a little TLC to get it going
It s in a garden after all  

An you can still watch it spin :  )

Everyone is giving great advice. You can see there are many possibilities with your project.  Your project also can depend on some other factors such as Local codes (Arg!), how much power you realistically wish to generate, how much money you want to invest in it, the possible noise any 'gearing' might make to nearby neihbors, and how pleasing to the eyes it will look in the end.

It looks like a very fun and beautiful 'artistic' first project for you and your cousin.  I personally love the looks of those water pump mills. I think most people do. It is very 'non-threatening' to the eyes.

Currently, I also am making something very similar to what you have, for generating, hopefully, 'pretty good' electricity.
Mine will be a SIMULATED water pump mill, but it has only 8-10 thin carved blades at a steep angle (4 degree blades) for much more speed than the high torque slow metal blades on your metal mill.

The blade rotor on mine is going to be about 6 feet diameter and directly hooked to a low rpm 'normal' dual rotor alternator (made W/small trailer hub)  such as made here on this site countless times. It will be about the same height as yours.

The simulated "water pump mill" I am putting up will look more like just a big 'yard decoration' with flower planters around it.  

Have fun with your project, keep asking questions, and always be safe.

Have read the following inefficient solution, but nevertheless one solution in an Oz magazine:

Leave the old wheel on and get someone to turn a wooden pulley for you. The wooden pulley needs to be connected to the wheel of the sails and will drive a generator from an old tractor. Not a modern alternator, which requires higher RPM. Such a tractor generator will turn like a motor when the batteries are connected, so you need a switch to only switch it on when the wheel is turning fast enough and is generating power. Or a better solution are diodes to ensure the current only flows one way!

The diameter of the pulley will need to be calculated, but I suspect it will be about a 4 : 1 ratio to the diameter of the tractor's generator. This generator will have carbon brushes which will wear out so the first thing you have to do is change the old brushes and perhaps also the bearings.

Not many watts but enough for a few lights and perhaps a lap top with a 12V interface. A large enough car battery should be OK for this experiment rather than a deep cycle variety.

Old mills like that need to be manually furled out of the wind when storms are expected, so watch it! Many a windmill lost its wheels during cyclones (hurricanes) in Australia.

Have fun in Malta!