I love lights! The worst thing though about decorating a tree is getting it up and decorated then turning it on only to find that some of your lights don't work! Plug your lights in before you even put the tree together! If your lights aren't working it will be easier to fix them if your not fighting an entire tree. If your tree is not a pre-lit tree then make sure you are using plenty of lights. A good rule of thumb is 100 lights per foot. Start at the bottom of the tree and work your way up. You are going to want to wrap the lights all the way to the back of the branch and back up so you can have depth to the tree.
The most important thing I can tell you to do is fluff your tree unless you buy a real Christmas tree then you can skip to step 2. Your tree has just spent a year in a box smashed and needs to be pretty again. This can sometimes take longer then putting on the actual ornaments but the result of a fluffed tree versus flat are worth it. I like to start one section at a time. If possible start with the bottom section and get it completely fluffed before you put the next section in. Fluff in different directions. You don't want all of the branches pointing up or down. Mix it up. This will help fill in the holes.
Are you putting ribbon on your tree? If so, now is the time to put it on. Ribbon can help reduce the number of ornaments you need to buy. It's easier to buy a few new ornaments a year then it is to buy for an entire tree. Use the ribbon to fill in. There are many different ways you can put the ribbon on. Check out the photo gallery if you would like ideas. You can spend as much or as little as you want on ribbon. If you are on a budget try Hobby Lobby or Michaels. They generally have ribbon 50% off.
Now you are finally ready for the fun part! Ornaments. I like to have different ornaments on my tree that I have collected over the years. To make this a cohesive look though you will want to purchase 10-15 (depending on the size of your tree) ornaments that are the same. For example 12 red balls, 12 gold stars, 12 green dome shapes, etc. Make sure you are buying different shaped ornaments so you don't end up with a ball tree. These ornaments don't need to be solid color. Some of my favorite's are multi-color that sparkle. Put these ornaments on first and put them deeper into the tree. Not every ornament needs to be on the tip of the branch. By placing ornaments back in the tree you will create depth and make your tree look fuller. Think of a pyramid when you are putting them on. You don't want them stacked in a straight line on the tree. These ornaments are also the perfect tool to use to fill in holes that fluffing could not fix. I like to make sure I have some larger balls that I can tuck in the back of the bottom section of the tree. This is where the holes tend to be the biggest. Yes, even on a real tree! Another great way to create depth is to find some mirrored or disco ball type ornaments. Tuck these in the back and they will reflect your lights and make your tree appear fuller.
If you have florals or picks you want to add you can put those in before or after the ornaments. If they are large florals I would suggest evenly spacing them before you put the ornaments on. If you are putting sprays in, go ahead and add these at the end.
Now go crank up the Christmas music and have fun!