You need to be looking at brokerage costs. There is a fee when you buy or sell and that can easily eat up any profit unless you keep the shares for a while until the price has gone up significantly. Regular buying and selling will just leave you worse off, especially in only small amounts. Your aim should be to research, find a company that looks good, then buy a significant number of shares and keep them for several years. Or else it will just turn into an expensive hobby.

Unless you have several thousand pounds spare that you can afford to lose, the best way to make an actual investment out of it is to go for a unit trust. This enables you to pool your money with others and share the fees. It does mean you're putting it all in the hands of a fund manager but there are hundreds of trusts to choose from. A FTSE index-tracker does best about 75% of the time and that's even cheaper because it requires no thought - the fund manager just buys shares in every company on the index. I've done rather well out of just doing that.

You "understand the risks and know where to get financial news from" yet you don't know how to choose a brokerage? Better study some more first.

go with fidelity or schwab, that is their main line of business

I'm a 20 year old hoping to get started with investing in shares. I understand the risks and know where to get financial news from. I bank with nationwide and they seem to offer the ability to buy shares. Has anyone done it through nationwide and can you recommend it?

If not, what is a good website to buy/trade in shares? I am an amateur !