Hi Nunesz and welcome to Guitar Noise
OK, the first thing you need to understand is that your Behringer amp was designed for guitar, not vocals. Guitar amps always add a little distortion, and that is what they are supposed to do. When electric guitar is squeaky clean it sounds lifeless and sterile. So, an electric guitar amp will always add some distortion, even on clean settings. When you can afford it, you need to invest in a PA. PAs are designed especially for vocals, you can get plenty of super clean volume.
That said, you can use the guitar amp for now. I would start on the clean channel of course. Turn Lows, Mids, and Highs to 12 o'clock position. Now get a distance from your amp and sing the way you will sing when you perform. If you scream, then scream. Now come up on the volume and listen carefully. It's really an ear thing. You may want to boost or turn up Mids a little, this is where the human voice is. If it sounds thick or dark, maybe come down on Lows a little. Adding a touch of Highs will give you a little clarity. But have a friend adjust the tone controls while you stay a distance away and sing. Don't worry about where the tone knobs end up, it's the sound that is important. Do little tweaks here and there until you get a natural sound, that has clarity, and good tone. It's all in your ears.
You might want to add a little reverb, that usually sounds great on vocals. If your Behringer amp has a preset for a combination of delay and reverb, try that. But don't overdo it on effects, too much reverb or delay sounds terrible, plus your vocals will not cut through the mix. Again, trust your ear.
The placement of the amp is important to prevent feedback. Never have the open end of the mic face the amp, that will cause a loop of sound, and you will feedback easily.
You might place the amp out in front of the band on one side facing sideways. This will enable both you and the audience to hear the vocals and prevent feedback.
Feedback will probably be your biggest issue. Have the band come down some. What good are vocals if you cannot hear them, or they are distorting and feeding back like crazy? Music does not have to be super-loud to be good, it is the groove
of the band that will make you sound hot.
If you know something better than Rock and Roll, I'd like to hear it - Jerry Lee Lewis