There are so many serums out there, for de-frizz, heat protection, shine...not all are good for all hair types, or even for what you think they are for. Hair serums can provide protection from the sun, smooth split ends, smooth frizz. Nothing fixes split ends but cutting them off, but serums can smooth down those ends and fake it till you can grow your hair a little bit more until you can get them cut. Serums are just a band aid really because they have silicone in them that coats the hair. You need to feed your hair with a good diet of healthy foods if you really want shine. What we eat and ingest and put in our bodies, is in our hair. So quit smoking, eat right, exercise and your hair will show it. Constantly putting serum on the hair will coat the hair, make it so it does not get nutrients and vitamins and oxygen. Start from the inside out. Serums are just a topical solution to a deeper problem.
If you want JUST SHINE...use a spray. Sprays are good for fine hair, that can't take much product before it weighs down the hair or makes it look greasy. Shine sprays do not do anything for hold, style or protection. And if you add heat to your hair after you spray, they give you that 'static' look. Like someone just rubbed a balloon on your head. It is just for shine. Spray and don't touch. Spray shine is more for if your hair looks dull, you want one more day before you wash it, or you just want it to look super shiny, because it will and its fun.
If you straighten your hair, you'll want a specific 'heat protection' or 'heat defense' that you use before you use the iron. When your hair 'steams' it will be the product you are steaming, not your hair. The anti frizz serums are better than nothing, but you should really use 'heat protection' serums. Remember that hair dryers are also HEAT. Put on damp hair before blow dry, apply more serum before the flat iron as well. The flat iron will 'seal' in the silicone and protect the hair, keep it smooth and keep those split ends smoothed. Ceramic plated flat irons and ionic irons and dryers are the best...as they dry from the inside out. Ionic means it emits millions of charged particles called ions that bond to your hair, breaking down moisture molecules with very little heat.
More about flat irons here This site is a great place to read and be able to decipher what all that stuff on the boxes means...'ionic' 'ceramic' 'titanium' etc.
Serums for curly hair are going to depend on the texture of your hair. Is it fine/wavy? Thick and super curly? You are going to have to try different brands. If its just for frizz, apply when soaking wet, apply again when a little more dry but still damp and maybe again in the day if you see frizz. All of that is going to depend on the weight of your hair and how much product it can take. Play with the amounts. Don't follow directions...put on dry hair if it just says for damp...use a spray if that works. There isn't one product for one type of hair.
In a nutshell:
Use sprays for lightness in product and super 'fake' shine.
Serums and oils for heavier hair, for de-frizz and for styling protection. Do your own thing...ITS YOUR HAIR. Eat right and have fun!