I am looking to mix/master people's stuff for free for practice
The easiest/best way to do it is like this.
Obviously make sure before you start recording that your voice is not going over 0 db.
What I actually do is like a 3 step process, I raise the volume then lower it then raise it again
This is what I mean by that.
First you record your vocals
Then send each vocal to a channel on the mixer so you can add effects for each vocal
I am only explaining the volume part since you are just want to know about volume....
Open up Fruity Limiter on EACH vocal. Raise the gain until the highest sound wave peek is just about to nip the green line at 0db.
Do this for all of your vocals to make sure your vocals are loud.
Now turn all the volume knobs on the mixer channels down besides the master channel. (oh make sure you have fruity limiter removed from the master channel because my Fl Studio opens up with it activated and I forget about it).
I am talking about the STRAIT UP AND DOWN volume knobs on the mixer, turn down the vocal channels all the way down.
Take the beat you are using and raise the volume or lower it to about -6 or lower db depending on how many vocals you have. The more vocals you have the lower you have to have your beat because your vocals will raise the volume if you have to many. If you only have a few vocals then just put the beat around -6 db or lower. It doesn't really matter just make sure its not close to 0db.
Now take each vocal one at a time and raise the UP AND DOWN mixer volume knobs until your vocals sound like they are mixed into the beat, but make sure nothing is going above 0db. If you have more voices to raise and your volume is getting near 0db then lower the beat further and restart the steps I just said.
After you have the beat and vocal volumes right you are almost done
Then go on the master channel and add a Fruity Limiter, simply raise the gain until the highest wave peek is just barley nipping 0db.
And its as simple as that..... your song should be pretty loud now.
Basically what you are doing is this
You start off by adding a limiter to all your vocals to increase the volume to the highest you can without clipping
The reason you do this is because you have to lower the volume of everything to make sure nothing reaches 0db, and if you were to lower the volume without doing this first, you would have a hard time hearing everything you are trying to mix since it would be at a lower volume, plus its just overall lower volume.
Then after you get your vocals and beat mixed right with the correct volume you raise the entire song with the master channel to make sure everything is as loud as it can get without clipping.
Pretty simple
Of course you have to do way more shit than this but you just said how to increase volume and thats the easiest way.
SORRY
I just realized you also said "clear voice", if you want a clear voice you will also have to add a EQ to each vocal channel and raise the high ends aka the right side of the EQ which will make your voice "Clear".
Its called Parametric EQ 2
The only problem is this
Your vocals will sound clearer but when you say words that have a "S" sound, the "SSSSSS" will be loud in your ears, so you will need to download a program to take off those SSSSSSS sounds.
Check youtube for a "Desser"
That is just about how to make something LOUD and CLEAR, but to have everything EVEN and SOUND good is a entire different story.