Not an easy question to answer with the limited information you've provided - I don't know if the UK has an equivalent to Crutchfield.com or the BestBuy chain, but I'd suggest you find someone local and have a discussion with them.
First - the speaker "power rating" (how many watts) is dictated by the output power of the device driving them, the head unit or if you're going to fit one, the power amplifier - most aftermarket head units have an output of 20 watts RMS/50 watts peak, so unless you're planning to add an amp, there would be little advantage to purchasing speakers that handle more than that, there's also the matter of speaker sensitivity, speakers that can handle more power are less sensitive (they require power to drive them).
On to your second question - does the car have provision to accept speakers in the door? If it doesn't you're looking fitting them will require custom enclosure fabrication, probably more than you're capable of doing on your own, and I suspect more expensive than you're willing to pay.
Moving on to the stock speakers - I have no idea what fits an Alto 2013 but it's not a high end vehicle so Suzuki would most likely have fitted a basic paper coned speaker with a whizzer, just about any aftermarket speaker of the same dimensions will sound better, coaxials will be easier to mount, we don't know what your musical taste is lime, if the speakers are less than a 6" and you like bass, maybe add a small active subwoofer under one of the front seats.
Lastly I want to point out that trying to install a system designed for anything more than "easy listening" in a vehicle like an Alto will require a considerable amount of work - cars are designed to move people, rather than to provide an ideal "listening environment", in fact, I consider the average car to be a very poor listening environment and whilst I will admit to having installed some fairly complex car systems, separate speakers, bi-amped systems with active crossovers, my focus has shifted, a decent after market head unit, decent speakers and a small active sub works for me.