I would suggest firing up Google Maps and finding some real world locations you like the look of. There's no need to copy what you see, only to use it as inspiration, and to get an idea for how landscape and topography affects the coastline.
Some basic points: in most cases, an estuary will form where a river meets the sea. On a map, estuaries look something like this:
Though of course you get all shapes and sizes. The Severn estuary, for instance, is hundreds of times larger than these examples.
In certain circumstances, deltas can also form at the mouth of a river. Deltas form when the flow velocity of a river drops dramatically after reaching a body of standing water, causing sediment to drop out of the water column. Over time, this sedimentation can build up, and the delta "progrades" out into the sea/lake. The classic example is the Nile Delta, though you can get all sorts of shapes - have a look at the Mississippi's.
Geology has a big part in defining the coast, so if you want, you can go into some detail thinking about that. The most basic rule of thumb is that softer rocks erode faster, so (in theory at least) if you have two rock types in an area - a soft rock bounded on both sides with a more resistant rock, a bay will be formed between two headlands. In practice it's much more complicated than that, but no need muddle things up further.
You'll also find flooded river valleys all over the place - fjords and rias. The Norwegian coast is the prime example but you can find them all over the place. There are numerous good examples in the South-West of Britain, for instance.
I think that's the sort of thing you need to think about. Try to come up with a more interesting shape for your continent, add in some estuaries, bays, headlands and islands, but don't crowd it out too much - have a look at some real world locations to get an idea of what's possible and what's not.
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