At the start of the campaign build as many trade ships as possible and send them to as many different locations as possible. When you get to some of them beware as there are usually pirates around and they will attack you. I had a ship sent out to the East Indies and it was chased by a pirate fleet. I withdrew and the pirate followed but did not catch me. I was then able to sail around the ship and land on the trade circle. Once you are there the AI does not attack you!
The other thing to know is that even if you place your warships around a trade circle they will not block the AIs trade ships from landing there. I made the mistake in one campaign of attacking the fleet there without having a trade ship to take it's place. By the next turn there was a Spanish trader there. Never again.
You will find that one or more of the trade circles can be held by a pirate fleet - simply attack that with your warships and have a trade ship ready.
Once you've grabbed as many trade circles as possible start to stack up your fleets. You can only have a single stack of ships in each trade circle.
The trade from that circle may not start until you have a trade agreement with someone that can trade there. I think in one campaign I was there for a while until I could trade (there may be no faction that had been built up sufficiently).
Once you've got your trade fleets in place make sure that you've got the warships to clear off any pirate fleets (or anyone else you are at war with) from sitting on the trade route and blocking them. The loss of income is hard to take when you expect around 20-30k per turn (early on of course).
The following are all from a campaign in 1826 but it gives a very good idea of what trade can do for income.
The finances 'blown up'
Trade summary - the longer you trade with someone the better the bonus is (the number shown is the bonus trading with France).