Party decorations have a reputation for being the stuff that lives in a crumpled bag in the basement for eleven months and then disappoints you in month twelve, half the balloons already gone soft and the streamers stuck together. I land somewhere more forgiving than that. The cheap stuff is cheap because it's meant for one afternoon, not because someone failed you, and once I stopped expecting birthday banners to last a decade I got a lot happier with what three dollars buys.
If you've got nothing yet, start with the plain stuff over the printed stuff: a garland in one or two colors, plain balloons instead of the character ones, a tablecloth in a solid shade. Plain mixes with everything you buy later, and a single "Happy Birthday" banner with no age or name on it gets reused for every birthday in the house for years, which the numbered ones never do. For sizing, measure the actual wall or doorway before you buy a garland or banner, not the table it's going to sit near, because the single biggest mistake I've made is grabbing a six foot banner for a ten foot wall and ending up with something that looks like an afterthought stuck dead center with acres of blank wall on either side. Hold streamers and garland up against the space before you cut or hang anything, you want it to read as full, not stretched thin. And check balloons for puncture spots with your thumb before you leave the store, because a bag of fifty with three bad ones is basically a bag of forty seven.
Where this earns its spot is anywhere people gather for a few hours and then go home: kitchens, dining rooms, a front porch for a graduation, a backyard in summer when the good weather means you can go a little bigger without worrying about tracking glitter through the house for a week. Smaller apartments do better with fewer, bigger gestures, one good garland over the table rather than decorations on every surface, because a packed room with low ceilings can start to feel like a closet with balloons in it. Houses with more wall and more light can take more color without it looking busy.
On finishes, I've learned to trust metallics, white, and one strong color over anything printed with a cartoon face, because the plain pieces photograph fine and don't look dated the next year sitting in the same drawer. Gold and white together has never once looked wrong in my house.
None of it is built to last, and you shouldn't pay like it is.