Owning a share means you own a part of the company.
Theoretically if the company is successful the value of the company will increase and the value of the share will increase.
Except it's not always that simple. The value of the shares will really depend on the demand and supply of that share.
Hopefully increased demand will follow success for the company - but will also depend on expectations.
Some shares are valued solely on market expectations - and can vary widely for no obvious underlying reason.
If you want to have exposure to the stock exchange then unit trusts are a good way of spreading risk with a small outlay - although you will lose out on management fees.
Alternatively there are a number of on-line share dealing sites that will allow you to set up an account and start dealing.
Usually the most economical amount to deal in is a minimum of around £2000. Doesn't mean you can't deal in less, just that you will be losing out on dealing charges.
Don't try to deal in shares unless you have money you are prepared to lose as it it is quite a high risk enterprise.
But it can also be quite fun and if you are lucky quite profitable.