hi pupuce
i have a couple - will put below what i can remember to the best of my ability.
an isa (individual savings account) allows you to invest a certain amount of money per year tax free. you can have either a mini or a maxi isa - different investment limits - the former £3000 and the latter £7000. the type of investment you can have in an isa is restricted to cash, equities (stocks and shares) and a third type (insurance?) which i can't remember off hand. you can have a number of mini isas (i think three) or one maxi isa an any tax year. you can check the info above (and i would given the addled state of my brain) with any isa provider (banks, building societies, investment houses, insurance cos etc). or you could try www.inlandrevenue.org.uk.
at the moment i've got a cash mini and an equities mini.
the cash one i just found from the best buys table in the cash section of the observer. they get their league tables from www.moneyfacts.co.uk. i don't remember paying any fee for this - it's effectively just a bank account. considerations in choosing one relate to (obviously) the interest rate on offer, whether you want instant access etc.
the equities one was a bit trickier. this is an investment on behalf of ds. i found a voucher for a free session with an independant financial adviser (again in the observer) and he basically said that before one considered any investment on behalf of children in their own name, it was sensible to use up one's own isa allowance. you also retain control of the money this way.
if you plan to invest in equities, i really would make sure you understand the risks first. they are really only an investment for the long term. i will not attempt to go into the risks here as i am not a financial adviser and wouldn't want to mislead anyone inadvertently.
please note the above comments offered in the spirit of shared knowledge/experience and NOT in the nature of financial advice!!!
hope that's a bit of help anyway.