Just can't get good at building?

LionessMN is absolutely right. There are tons of tutorials on building out there. For an absolute beginner, I recommend the Out of the Box Building by AdelaideBliss. It's short and very basic, but it's a good starting point.

Rosemow started a thread that is posted above full of tutorials also. You can find it Here.

When designing a new house, I start with looking at real life blueprints. You have to use them with a grain of salt because the proportions can tend to be a bit off. The general rule of thumb is 3 feet=1 square. I tend to round up, but be prepared to make smaller or larger to fit what you want to do. Also, for your roofing and exterior colors, look at pictures of real houses. Look at contrasting colors, accents, etc. Ask yourself questions like what do you like about this house and why. See if you can duplicate designs you like in Sims 4. Also, look at builders on the gallery and see what draws your eyes. What do you like and how did they do it? Never directly copy, but see if you can replicate styles, or pick things apart to figure out how it was done.

Learn the keys and cheats that will help you. They are listed all over the place, but for example, the Alt key is your friend. Used properly, you can move and place things where you want. Learn how to move items up and down, rotate freely, place freely, enlarge things or make them smaller.

With roofing, you can hit Shift while you adjust the eaves to change just that side. Also, roofs can stack if you have turned on move objects, so you can use them to cover areas more easily from different angles. And I think it's CTRL + SHIFT + C that will add the extra little markers on the roofs to change the shape. Hold down ALT when raising or lowering a roof pitch. This allows a smoother move and less snap to grid type of change of height (hard to explain that).

Test different swatches for different designs and colors. When looking at other's work, I'm always surprised to find things I didn't know existed because I never looked through the swatches completely. This goes for walls and floors too. Recently, I've used the sort tool (on the bottom left of your screen, to sort by colors. It's not the best feature, for example, if I know I want a blue wall, I can sort by blue and it will bring up all of the options that have blue in them. I have found a lot of stuff that I had missed before.

The absolute best advice I can give is to just keep trying. Build and build and build more. Every build I do, I learn something new. Also, sign up for an image hosting site, such as imjur and take screen shots. Post them here. People here love to see other's work and you can ask for feedback if you like. Join shell challenges. Most are for just fun and to showcase what you have done, but you can get some good feedback there.

Understand it's like any art. You will have to keep plugging at it to get truly good. Have fun with it though. Make designs that you enjoy. Give different techniques a try to find new ideas.

Most of all though, have fun and keep trying.