Before you start to build, you need to pick out the blocks you'll use. A block palette should be made up of 2-3 harmonizing blocks, 1-2 contrasting blocks, and 1-2 detail blocks. Here's an example.

In this block scheme, the wood and logs are the harmonizing blocks, the stone bricks and stone slabs are the contrasting blocks, and the leaves and fences are the detail blocks. Here are some more block schemes that I like.

Here is something I'm sure all of you have seen before: a blank oak planks wall.
I know, boring. Let's add some detail. But first, we have to pick out a block palette.There we go! Now we can start to add detail. Having pillars in front of the main wall adds depth, like so.Stairs and slabs can make a wall or building look more sturdy.
Let's add some windows/arrow slits:
Here, I added a bit of white wool, to add a bit more detail.
Bushes and flowers will always be your friend. Always.
And now we have a completed wall! This is kind of a medieval style wall, but the technique and tips can be used in other styles.

Minecraft is mad up entirely of squares, so obviously making curved shapes is going to be a bit difficult! When building curves or arches, don't make a section longer than the one before it. In this arch, the section lengths go 4, 3, 2, 1, then 1 again. You can also build backwards arches using the same rule, but starting with 1 and getting bigger, like this:
Circles are basically just four arches connected to each other. Here are a few sizes of circles:The bigger a circle is, the more circular it will look.

There are a lot of different styles of building. They all have different block palettes, and they look good in different biomes. Here are some of my favourite styles of building.

This is a steampunk theme. This house is based off of a build in this guide. Steampunk is my favourite build style at the moment. This style looks good in savannas or forests.I'm not quite sure what to call this, it's kind of a log cabin theme I guess? It would also look good with red, orange, yellow, or gray stained clay. This style looks good in taiga biomes.This is a medieval style wall similar to the one I used in the Depth and Detail section. This style looks good in forests.
This is a middle-eastern style house. This style looks good in mesas and deserts.This is a modern style house. Modern is one of my favourite styles to build in, after steampunk. I based this house off of a build whose link I put in the comments. This style looks good in plains, forests, and deserts.

Before you start working on the interior, you need to pick out another block palette for your furniture, carpets, and other things. Here are some furniture designs I particularily like.
Toilet
Shower
Bed
Desk
Couch
Armchair
ChairCurtains
You can also create your own furniture designs. Some good blocks to use are stairs, slabs, carpets, and fences. Don't be afraid to experiment, you can always take it down and try again.