Soo I just entered a competition in my school for making a digital posters, and it’s also my first time, Soo I didn’t know anything about making a posters…
And the fact that I’m still not good at art and the lack of knowledge of mine on posters makes me even scared…
Soo I’m posting in hoping I get some tips n trick!! 
Maybe like do and do not do things on making posters~
Any help appreciated!!
Generally, making an artwork for a poster is not much different than every other. However. when it should be printed, you need to make sure the colors you choose also work in print, since a screen can show much more colors than a printer can print.
Here is a detailed explanation:
Long story short, use Krita’s soft proofing and out of gamut warning to keep the colors in a range a printer can print and what also looks good on a screen.
Also, keep in mind that when you don’t have a calibrated display your artworks can look a lot different on someone else’s screen.
Keep in mind the dimensions you’ll need. 300ppi is the golden standard. You can get away with 150ppi but for your usage I’m guessing it’ll be inspected up close so go for 300ppi if you can.
Use bleeds (extend the artwork beyond the margins). Some printing places will support them, and bleeds also let you move things around a bit to choose the best cropping for your chosen composition.
Working in RGB tends to be fine, most places handle the conversion from it to CMYK just fine nowadays, but check how it looks in CMYK just in case.
Check your work in different displays. You’re likely not working on a calibrated one, so getting a consensus on how it looks in different computers and phones is the next best thing.
If it’s not a vector you can slightly sharpen your artwork to make it look better when printed.
If you really want it to print okay you can fill a test print sheet with shrunk down versions of your poster with darker and lighter colors, sharper edges etc. In my experience prints tend to look darker than the display, but it may happen they come out washed out. Only test prints will tell you what it will look like when printed by a given service.
Start in a small resolution that your computer can handle and only resize to the final resolution when you really need sharper details (50% done or more). You computer will thank you.
If you’re on Linux using zramdisk might be helpful. I was experiencing constant freezes on Krita at very high resolutions when my system started to swap to disk and it seems to have solved the issue.
Maybe also look at other software since graphic design isn’t a strength of Krita.
I mean - I would use Krita for any drawn/painted imagery, and maybe basic text tasks, but for vector images (good for scaling to large sizes) and more complex text design (also vector-based), I think Inkscape is the main open-source option.
For photo editing, Krita is okay but GIMP is more fully featured. Actually - there are quite a few options for FOSS photo editing. Darktable is another one I’ve used that’s very good.
In terms of actual design, I would say be careful not to clutter the image - you want things to be easy to read (both text and imagery). Use contrasts to make things pop - light against dark, or bright colours against less saturated areas. You want it to be noticed at a distance and draw people’s attention.
It would be best to explore your ideas through sketches, and thumbnails first. Then you can develop your ideas and solve composition problems before you invest lots of time in more refined work.
Oh I would love to, but sadly I’m a android user