So I just bought Riven and was wondering if my build is good or if I should change it a little.
Start with boots and 3 health potions. First trip back I get dorans blade. After that I either finish my boots or get BF sword. then its like this bloodthirster>warmogs>atmas impaler>maw of malmortius>bloodthirster.
So I end up with :
ninja tabi/merc treads
bloodthirster
warmogs armor
atmas impaler
maw of malmortius
bloodthirster

I don't do this everytime but this is what I do for a balanced team.

I'd switch out the Warmogs for a Frozen Mallet, but your build seems acceptable. Maybe ditch the second BT for a Guardian Angel.

Why do so many people say to get guardian angel for riven?

she tends to die in the middle of team fights, thats the nature of Riven, get in there and do stuff, then when you come back you can Ki strick, and use your Q to finish someone off or use your E to get outta there quick fast and in a hurry.
But if you ditch the warmogs, which I agree with, then the atmas is also less useful, so you might be able to build another damage item, IE or or banshee's veil depending on the situation.

So ditch the warmogs and get a frozen mallet and get a guardian angel instead of the atmas.
So I will have
ninja tabi/merc treads
bloodthirster
frozen mallet
guardian angel
maw of malmortius
bloodthirster

would that be better or ditch the maw of malmortius for something else?

Alrighty, I main Riven. She's the only character I'm really good with tbh, lol, but here's some advice from a noob...
Frozen mallet will give you a nice advantage in chases with the slow.but it doesnt really have the health that WM has thatd make an Atma's viable or efficient. SO either WM and Atma's, or maybe FM and Banshee's. And, of course, it's situational. IF the enemy is ap heavy, Banshee's. If theyre balanced, pick up Guardian's angel, which really helps to keep Riven's snowball rolling (she has one of the largest i know of). AD heavy, Thornmail, or Atma's. I usually go atma's and warmogs cuz it makes you tanky and gives good damage on teh side, but yah. Your build is just fine.

Just some advice: If you're losing your lane, or your against someone that you need to build some armor against, buy an early wriggles. The armor/sustain will help you get back on par in lane, and the free ward always helps.