Think ribbons to add bulk to a tree, if you don't have a lot of decorations. And lots of lights!
Kinwoven designs on Youtube have lots of Christmas decorating tips and tricks, they have a great video on making garlands look so much better with extra lights and bits of fake greenery/foliage ("picks") and baubles etc. added to it rather than worrying about the original garland itself being fantastic. (She does Christmas BIG, and has spent more than 20 years and a LOT of money gathering decorations so learn the tips, but don't feel the need to go overboard buying).
We started with a small tree, 4 lengths of tinsel (ribbon wasn't a thing then), 1 bag of 20 cheap red baubles and a couple of stockings I made in red and green felt (about 3" long, and I used the ribbons I cut from tags and other clothing loops I didn't need for the loops to hang them) and some pine cones dipped in white paint with string loops to hang them. We gathered other decorations and baubles over the years on our travels and in lots of different stores.
Pick a base colour or 2 coordinating colours this year - and either gold or silver to go with that. You will find lots of packs of baubles in so many colours, so get 1 decent pack (preferably with smaller to larger sized balls) as your base. Get a ribbon that will work with that to make bows or to thread through the tree in loops. And add a handful of nice individual decorations that you make or buy.
It's easier if you have that base colour decided, and the accent of gold or silver, as you'll see so many things in shops but keeping to a basic colour palette and theme helps. Red/Green and gold works well as a starting point and is quite traditional so there will be lots available, but you could also do more woodsy/natural vibes and mellow with brown/white added to the reds/greens rather than gold/silver. But I also love blue/white and silver (and have accents of turquoise/greens for a sea theme some years, or some pink robots others).