You have all the pieces .. motherboard .. cpu .. ram .... heatsink/cpu fan?
Place the cpu into the motherboard socket. Note that it only fits one way. Check the pins on the cpu against the holes in the socket. Hold it in place with the securing catch/lever that you will see on the side.
Smear some heatsink/cpu paste onto the top of the cpu. The heatsink paste is basically a 'heat resistant glue'
Put the cpu cooling fan onto the cpu and screw it into position. Or, it may have a securing lever to hold it in place.
Put the RAM into the ram slots. They only fit one way. Make sure they are firmly in place.
Put the motherboard into the casing. Align what holes you can and use the screws that come with the casing to secure the motherboard.
Connect the casing power supply to the motherboard. Note that it only fits one way.
Connect the cpu cooling fan to the motherboard connector. It only fits one way ..
Next, look at the leads that are just lying loose .. the small female 2 pin connectors will be marked 'PWR' ..'LED'..'RESTART' ... There maybe 8 of them.
On the motherboard identify the block of male 2 pin connectors. The female connectors must connect to them. You'll need good eyes and probably a flashlight, as the writing on the motherboard is small..
If you are connecting a sata hard drive, connect to the motherboard connection that says 0. Connect your dvd drive to '1' Again, notice that sata leads can only fit one way.
Then connect any video or sound cards to the motherboard (if any).
Just look on the motherboard for anywhere the connecting wires will attach. You won't go far wrong.
The casing won't come with instructions of 'what to do' as such. If there were instructions there it would relate to pin/hard drive connections.
What you CAN do is a google search. If, for example, your motherboard is an ASUS 123456 .. search for 'ASUS 123456 motherboard connections'. Or log onto the motherboard manufacturer's website ..
Here's a couple of links that may help:
How to Build Computer The Complete Guide
How to build PC step by step comprehensive guide
How to Build Computer