Because I am not able to get down any glitches that are used for smw speedrun I wanted to ask what the best way of learning a glitchless run.
I have looked for the category, but somehow I can't find it.

Just try to find the fastest route for each level that's in your comfort zone. That's all I can say.

Do you know how much time the orb glitch and the cloud glitch saves?
Just to know if it is worth trying really hard...

Orb saves around 15 seconds in World 1, and cloud saves about a minute in the Bowser fight. Going glitchless is a perfectly valid option especially if you're just starting out.

To submit a run as glitchless all you need to do is select that as the 'Run Type' when entering the run details; this is only an option for 11 Exit and its no cape variant so if your run isn't one of these just submit it the same way as any other run

Are there any leaderboards for glitchless?

I thought it was a fairly big category...
Are there any other websites Wich show glitchless leaderboards?

In the 11 exits, you have to select "Filter" and select "11 Exit run type - Glitchless" and you will have a "Sub-Leaderboard"