An altar or shrine is like a crossroads, in some ways; in others, it's like a bird feeder. It's a place where our world and the Otherworld interact.
When you're building a shrine, you're creating a place that has things the deity or spirit in question like, desire, and might in fact need to be present on the physical plane. If you create a generic space, you might get whatever comes by attracted by the not-so-specific energy in the area; because of this it's always best to create shrines that have a really clear intent and energy.
Not sure how to get started? Research is great. So is asking questions! Mythology and history show us lots of different symbols, objects, and colours for the Gods. If you get stuck - ask Them. Ask them what they'd like. They're not shy and will usually show you things. Sometimes things will literally just fall at your feet.
Don't be afraid to do it "wrong". You do have to think, and prepare, and not half-ass it - but as long as your intent is clear, you should be just fine. If you put something on a shrine or altar, and whomever's shrine it is doesn't like it? It'll break. Or fall off and roll under the couch. Or come off the wall. Seriously. If They don't like it, they will definetely let you know. So approach shrine-building without fear, but come armed with knowledge and fore-thought.
Altars and shrines are living things; they have energy flows, like anything else. Don't let them be stagnant! Add things, take things away, change the way they're set up, let them grow or shrink as they desire. Touch your objects. Play with them. The more in-tune you are with your ritual and magical spaces, the better.
Keep them clean, and well-fed - ie. supplied with fresh candles, incense, food and drink offerings. A dusty shrine is like a house in bad repair - it discourages guests, and if you're keeping the space in a particular deity's name, can be very offensive to them. It says to them that you can't be bothered to treat them with respect, and that you aren't willing to honour the commitment a shrine is.
Shrines need to be tended. If you can't make an offering, light a candle or incense, meditate near it or pray at it, or even just make sure it's clean at least once a week, it's probably not a good idea to build one. Some people like to build shrines on a more temporary basis because of this - have an empty space set aside for it, build it when you want to use it, and then store it again when you're finished. This doesn't build up the same kind of energy as a fixed shrine, but it does work.
A shrine is a meeting place. Greet your Gods, or spirit allies, every day! Keep it well, and you'll always have the Gods with you, wherever you are.