Making coffee can be a simple process, but there are plenty of little things that you can do to make a bad cup. Though I'm presuming you'd rather make a good cup of coffee, so these might be a list of things to avoid doing.
The most common way to end up with a bland cup is to use coffee that has gone stale. Roasted whole beans will stay reasonably fresh for up to 1 to 2 weeks, if kept in an air-tight container. Ground coffee only stays fresh for a few days.
I like a bit of milk and sugar in my coffee, but too much of a good thing will turn even an excellent cup of coffee into a watered-down and sugary mess. These extras should accentuate the coffee's taste, not mask it. When trying a new coffee, you should have at least one sip without any sweetener, to appreciate the taste.
If you get your beans for a decent coffee house, this won't likely be a problem. But if you are roasting your own beans, make sure you don't go overboard. More roasting doesn't mean more flavour. Roasting only takes between 10-20 minutes. Burnt beans will make bitter coffee.
Coffee brewing methods that involve boiling your coffee (like using a stove-top percolator) can result in poor coffee. The high heat can destroy the delicate flavour oils in coffee, leaving you with a pretty unappealing cup. Try using a drip coffee maker instead.
5. Letting Brewed Coffee Sit on the Warming Plate
The warming plate of a drip coffee maker will keep coffee warm, but forever. You'll end up with more stale coffee if you go for a cup that's been just sitting there for hours. Take a few minutes and brew a fresh cup.
If you're using a drip coffee maker, a paper coffee filter can taint the taste of your coffee. This can be a particular problem with cheap filters. Reusable plastic or metal mesh filters are less likely to spoil your brew.