I've just started playing OpenTTD (Thanks to Macha's blog post) and most of my train and cars are making a loss (or not much profit) how can I tell how much money the business will make? And how long they'll take to make it.
It isn't possible to tell accurately how much a service will make. A train makes money by delivering cargo, so the amount of money it makes in a given period of time depends on:
More information on how it's calculated: http://wiki.openttd.org/Cargo_income
To make money initially, I find a coal train from a coal mine to a power plant about a screen and a half away with 3 or 5 carriages is nearly always profitable.
It's not possible to tell, but you can make an educated choice on the services you implement to increase the chances that you do make money.
For example, taking advantage of the subsidies offered is a good way to boost your income early in the game.
Also, I usually start with some simple passenger bus services between two close towns. They seem to be profitable for me just about every time.