Hey y'all. Very important question here.

Our practice studio is 30m² (98² feet).
Here is how the gear is placed :

I am the singer, and very often I can't hear myself sing properly, or the other instruments.

What is the best way to setup everything ?
I tend to setup like a live situation,
Monitors facing the band - we have 3 though 2 for front and an extra one for Mr Drummer man
Drums at back
All Amp lined up to their preferable side at the back
If you can't hear your singing...or the instruments...then I'm afraid to tell you that you've lost your hearing...

But seriously, the two things that people at band practices need to learn the most, is to learn how to use the P.A properly, and how to turn the volume down!

I'm to assume you know how to level the vocals properly and I'm just going to suggest that the instruments need turning down.

I think the fact that the drums are right where the vocals are coming through hurts the sound.
Usually when either of my bands practice, the drums are at the opposite side of the room to the vocal speakers.
Drums are the loudest instrument usually, as there's no real way of turning them down without changing the playing style a bit, so to put them where the vocals are coming through could cause problems.
We don't have any monitors

I we always try to play quiet but still!

So the thing would be to place the 2 vocal loudspeakers opposite the drums then ?

And the amps ?
Well I don't really know how to use it so I just use the volume level. there are a lot of know like gain, "HF" etc...
what my band does, is set up just like a live situation. But instead of having the PA pointing out, its being used like monitors so everyone can hear the vocals well. the guitar and bass amps are loud enough that they can be placed behind the players, but the drums are the farthest away from everyone, since they're the loudest.
Guitars: Epiphone Slash AFD Les Paul, Schecter Omen-6, and Fender CD-60 Acoustic
Amp: Jet City JCA20H with JCA12s cab
Pedalboard: Crybaby Classic, Boss DS-1, Korg Multi FX, BBE Boosta Grande, Rocktron HUSH Pedal. ProCo Rat.
The problem is you are playing in what is a very small space for the gear you have.

The first thing is you are probably too loud, We all have the same problem. A drumkit in a small room is difficult and if you all play at the same volume as the drums it really will be too much. Your ears actually limit the way they transmit sound to the inner ear at high volumes so you lose detail. the second problem is that all your amps are pretty directional and the small room means you are pretty close to them. they probably aren't pointing at your ears, more than likely they are pointing at your knees, which don't hear so well. Put your amps on chairs or something and tilt them back to point at ear level.

Turn down as low as possible, if you can't hear someone then turn everyone else down and avoid the temptation to keep turning up. Put bass and drums next to each other and the vocal PA/monitors as far away from them as possible but pointing towards them across the room. Guitarists absolutely have to have their amps pointing at their ears as they have the most directional amps and they will be too loud if their amps miss their ears.

Hope this helps