I don't know how much you know so I'll start from the top (I work at William Hill so I have to explain this all the time lol)
On Saturday we are 6/4 to win with Ladbrokes. This means if you put £4 on you will get £6 back in profit, and the original £4 you put on, so £10 in total. If you put £10 on us at 6/4 you will get £25 back (£15 profit, and the original £10 you put on).
To work out your returns its pretty simple. You Just add the two numbers together, so if its 6/4 you do 6+4=10. Then times the 10 by however much you want to put on (in your case £10), so 10x10=100. Then divide this number by the SECOND number of the odds (so if its 6/4, divide it by 4) so 100 divided by 4 = 25.....so you'd win £25!
As for where to bet, doing it online usually get you better prices. I did an accumuator in a shop a few weeks ago which returned me £60, if i had done it online I would have got £90! Pretty much every website has offers of free bets when you open an account, so look about for the best one! But I use Ladbrokes because a) they have an iphone app, b) their site is really easy to navigate and c) their minimum deposit is £5, most others are £10.
Hope this helps, and most importantly, I hope you win your bet lol!!