Get the case on it's side first and install the motherboard stand-offs. Then plug in all the modular PSU cables you'll need into the PSU
before installing the PSU into the case. Install the PSU. Push the motherboard backplate into the back of the case making sure to lift the little metal flaps enough to go over all the connectors.
Take the motherboard out of the box and anti-static bag. Place the anti-static bag on top of the motherboard box and place the motherboard on top of that. Install the CPU, taking care to install it square into the socket. The CPU will only fit one way, check the locating lug holes on the CPU substrate match the locating lugs in the socket. Install the RAM, checking the correct slots to use in the motherboard manual and orientating the modules with the locating lugs correctly (it's usually slots 1/3 or 2/4 needed when using more than one DIMM). What some people recommend is you keep the motherboard outside the case, install everything I mention below and see if the system starts. I've never bothered to be honest. I only take it back out of the case if something doesn't work properly.
Now is also the time to install the CPU cooler. You going for an air cooler or AIO water cooler?
Once all that is done place the motherboard into the case and line up the holes to the stand-offs previously installed. Screw the motherboard in, taking care not to over-tighten the screws.
Now install all the required SATA data cables into the motherboard and start routing the PSU cables around the back of the case to where they are needed. Now is also a good time to install the front case connectors onto the motherboard. This is always the most fiddly, PITA task when building a pc. Check the motherboard manual - it'll have a guide on where to plug in the case power switch, reset switch etc. It'll give you a cable orientation using '+' and '-' symbols and may also use the term 'ground'. The '+' side is usually the coloured cable but to be sure check the back of the little black connector that plugs into the motherboard - over one of the cable entry points will be a small triangle.
This is the '+' cable.
Now you have that done you can go about installing the operating system drive. If your build will contain more than one hard drive just install the operating system drive for now - makes life easier, trust me
Install the GPU, remembering to plug in any power cables needed and ensure the card is fully seated and secured into the case with the 2 screws. Start with one GPU for now - we just want to ensure the build powers up.
Double check all power cables are in - 24pin ATX motherboard, 12v 4 pin (or 8 pin depending on the motherboard), SATA and GPU.
Now, connect the GPU to your monitor, press the power button and it should start up. If it does then you can go into the BIOS to change boot order, ensure AHCI is selected etc. Once happy then reboot and install the operating system.
If it doesn't power up then come back and we'll help you.