If there is any basic accessory almost every photographer should own, it's a 5-in-1 reflector. Versatile, portable, and cheap, I have one in my bag at all times. This excellent video will show you the many different uses for a 5-in-1 reflector and how you can employ one to improve your shots.
Coming to you from Vanessa Joy with Adorama TV, this great video will show you the many different uses of a 5-in-1 reflector. 5-in-1 reflectors are incredibly versatile accessories, as they can be used to flag light to increase contrast, diffuse hard light to soften it, reflect it to fill in shadows, increase specular highlights, add warmth, and so much more. They do tend to get misused a bit, though (people tend to default to using only the silver side, which might not provide the result they're actually looking for), or they often position it improperly. Along with its versatility and price, that's why I believe a 5-in-1 reflector is one of the best tools a new photographer can use to learn how to shape light, and it will continue to be useful as you become more advanced. Check out the video above for the full rundown.
I'd highly recommend checking out the 5-in-1 reflector from Pixel Addix. I've had mine for about 2 years, and while I've seen others with a handle, it's the only one I've found with a 1/4-20 mount in the handle. A life saver when you're working on your own. Purchased from Amazon for under $20.
The Lastolite Halocompact is the only smaller reflector/diffusor I use. Very portable and very sturdy, do not flex and can be mounted very easily on arm due to 1/4" thread in the handle.
One thing to look out for when you buy cheap reflectors are the fabric is often treated with bleach or any other detergent, which will give you all but neutral color. this is easy to test by using a UV light on the fabric. Also, the metal frame is often not of high quality and will deform over time or cut thru the fabric.