Only put money into stocks and shares that you could afford to lose. Risk levels vary, of course... but you'd need to be comfortable that things may not turn out well. If you decide to invest in shares, then unless you know what you're doing (i.e. you're able to track share prices and be ready to sell and buy as the market moves) then I'd suggest it may be worth looking at a tracker product that spreads the risk (i.e. your cash is invested into a wider raneg of companies so that if one or two go down than - hopefully - the others will make up for it).
ISAs can be tailored for plain cash or stocks & shares... the safest (i.e. least risky) option is cash but the interest currently paid is very, very low. A stocks & shares ISA just means that your profit gains are paid tax-free.
Over the longer term (perhaps, say, five+ years) the stocks & shares option will usually out-perform a plain cash saver. In a dream scenario you'd probably be best to save a proportion of your cash into a Cash ISA, some cash into a low-risk shares tracker and, if you're willing to accept a higher degree of risk, a bit of cash into a more high-risk tracker.
But... I'm not an expert at all. I lost cash in the past by foolishly investing predominantly into shares in my employer. Safe to say that I learnt a hard lesson!