Help me with this rack im trying to build,currently i have bought the arturia 3u rackbrute

OK what are you trying to do? what type(s) of music do you want to create? what other equipment do you have, that can be used with the rack?

I'll say this on the assumption that you have done near 0 research - return the case if you can and get a bigger one - tiptop mantis or doepfer lc9 are good starter cases - because that way you won't be buying another one in 3-6 months - better to start with a big case and fewer modules than a nearly full small case!

often recommended starting modules are: a sound source, a sound modifier, a modulation source, a way to play and a way to listen - including/plus a quad cascading vca (which may be your way to listen) and a utility starter set of something like links, kinks and shades - I would try to follow this more closely

Utility modules are the inexpensive, dull polish that makes the expensive, shiny modules actually shine!!!

Jim is once again spot on, you need a bigger rack. Eurorack requires lots of space as these single use modules take up space and you need a lot of them to get the most out of your investment. If you buy a small case because the bigger cases are too expensive you are setting yourself up for disappointment.

There are lots of cool videos of small racks sounding awesome, the issue is that these small racks sound awesome for that specific sound and in some cases only that kind of patch and without enough supporting modules you will get bored or frustrated rather quickly. Ultimately either reselling your modules or getting a bigger rack that you should have started with.

It would be in your best interest to start with a bigger rack (don't focus on the need to fill it right away) so you have the best chance of finding your way in eurorack. It is an expensive hobby and there are lots of other ways to try a little before making this kind of investment like a neutron, dfam or similar semi-modular system to start.

I also wouldn't start with the Ataraxic (I have this one) as a voice, this is apologetically digital and taming it into warmer tones takes a fair bit of work. I would start with something like the Dixie II (my favorite workhorse osc), or similar analog osc. I like the Dixie because it has a ton of features at a great price, you will get a lot more mileage out of the Dixie as a first osc imho.

And another heard from on the rack size: it's too small. The 3U Rackbrute is better used to expand the Minibrute by adding a few things that it doesn't have. But to do a proper build, you'd need to start with the 6U version...and I'd even think that THAT was a bit small unless you're good at using alternatives to the full-sized modules.

These 3U builds simply DO NOT WORK unless you're designing something very specific that can fit into that small a space. Otherwise, you'll tend to run into that space limitation over and over. Yes, you see these on YouTube all the time. But those are either done by very experienced modular users OR they're unworkable pieces of crap cobbled together by someone who needs to stay the HELL off of YT and stop misleading people.

"someone who needs to stay the HELL off of YT and stop misleading people"
LOL, I can think of a handful of candidates to that...

btw.... there is nothing stopping you follow your own path, though keep in mind some of the feedback from this forum which 'may' help guide future choices of gear.
It can feel like they are here to "Rip you a new one" but I'm sure they are all lovely xxx and actually trying to help.

You could start out with quite a lot less and still enjoy what you make before progressing to more modules.

But yeah.... Don't get into Modular, It will really screw *you up...

*Bank Balance.