The high street bookies broadly speaking are pretty much of a muchness. Though I tend to find Paddy Power usually the most generous with their odds and money back specials they do sometimes. It depends what's in your area too.
For the world cup, they will all have separate coupons for each match. The odds for a Brazil win tonight are around 3/10 which isn't great, 30p back for every pound staked in other words. Croatia are around 12/1 to win and the draw is around 4/1.
So you could put a fiver on Brazil to win and get back £6.50. I personally wouldn't bother with such poor odds. You could go out on a limb and back Croatia to win but 12/1 is a long shot in outright match bets. 4/1 for the draw isn't so outlandish and even though Brazil are clear favs to win tonight, these opening games don't always pan out as forecast. Expectation of the home support may get to the Brazilians.
Predicting the actual scores is obviously harder to do but the odds are much better than outright win bets. Brazil to win 2-1 is 17/2 or to win 3/2 is 45/1 (Paddy Power). Then there are other numerous bets they have on the coupon. Winning at half time and full time and so on. First goalscorers, will both teams score? etc etc. Then there are accumulators where you are predicting a set of games, so Brazil, Mexico, Spain and England all to win their opening matches for example.
Just go with your instinct. Pop into your local bookies and have a look at the coupons and have a go. It's a bit of fun and good luck!
Just to add that the online odds that the bookies quote aren't always the same in their corresponding high street shops. They tend be to a bit lower in the shops.
Finally if you google Oddschecker. A comparison website for all the different bookies, both in the high street and the online only ones.