Is rooting a surefire and safe process, or is it risky? Is rooting my tablet safe or something can go wrong with it?
Is rooting legal?
Does my device become slower if I root it?
What is the best rooting tool?
Is it possible to lose my data or something if I root it?
"is rooting safe"
The answer is difficult. Every tool you use to root your device says do with your own risk and we are not responsible for any damages caused to your device.

is root legal
Nothing to do with legal but it voids your devices warranty.

does my device becomes slower
No , many tools and tweaks are available to speed up rooted devices .

what is the best rooting tool
That depends on what device you use.

about data loss
No data will be lost. However a backup is taken to avoid data loss.

Try googling to find the best rooting tool for your particular device. Alternatively, 
you can download some rooted stock roms (specific for your device) and flash them using recovery.

Try e.g. Kingo root, this is the best for me.