Hi, i'm new in Linux but i'm eager to learn more. I've chosen Debian as my main distro, and i came across a slight problem.

I have an SSD 250GB in which i have Windows installed and a flash drive in which i burned the ISO file using Rufus.
After doing so i've been able to go through all the easy steps of configuration although i don't know how to format my SSD in the 'Partition disks' section.

I've booted into my flash drive.

https://imgur.com/fI1z3EU

Can anyone offer any assistance?
Thanks in advance.

Shrink the 245gb partition to give yourself some free space for linux to work with. Windows disk management is the best way to do this. Maybe give yourself 100gb of free space. Then create partitions with that free space as needed for linux, 15-20gb for root and rest of free for /home or /data

The problem is that you only have just under 3MiB of free space available as far as I can see from your screenshot and therefore, you would need to shrink your Windows partition(s) to make room for Debian. Then you could format your newly created partitions for Debian.

So i would boot into windows, shrink it, create a new partition, place Debian into that allocated space and only then would i be able to format windows?

Yes, you could do that, provided that there is enough unused space on that partition - as in: the files on that partition don't take up more than 147/148GiB of space on that same partition.

That's only partition you could resize/shrink anyway by the looks of it.

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So this is what i have:
https://imgur.com/KH7ngdD

Should i use it as Ext4 and the mouse point as root? I'm not sure how to procede from here because bootable flag doesn't seem to want to turn on...

ext4 is a safe choice in relation to the filesystem you wish to use for Linux, and it's well supported in Linux.

It's up to you exactly how you partition the drive for Linux now it seems you have resized your Windows partition. Personally, I create a "home" partition, a SWAP partition and the "root" partition (where the OS/Linux itself is installed to). You'll need a SWAP partition if you plan on being able to hibernate your system in Linux.

I let the installer program for my distro of choice (being currently CentOS) worry about making my "root" partition "bootable". Never do it myself manually.

Let's say i'll divide the partition that i've created into the Home,swap and root partitions. What sizes would each partition require in average?

Since i've created a partition there's no real necessity into doing this manually right? I could just do guided and place everything in one partition?

In relation to the SWAP partition, it depends on how much RAM/memory your machine has. Let's say it has 8GiB of memory, in that case the SWAP partition should be the same amount (8GiB).

In that above example you could then allocate 40GiB to the "root" partition, and the remaining space to your "home" partition. The primary factor in deciding the amount of space for your "root" partition is how much space the Debian install plus whatever else you want to install after OS installation takes up. Make sure you don't make the "root" partition too small, otherwise you'll have problems later on. Up to you.