Hey guys! I currently have a mountain bike which i just setup my self. It has wheels and everything just except the chain and pedals. But my biggest concern is that i took out the gear shift, but still have the multi speed chain ring and cassette it already has. I was planning if i was able to just put a chain and it would just work on one gear. If i cant do that do you know how i can make it like that without changing to single speed cassetes or chainrings? i currently have no moneyy :[

Thankssss!!!!!!!

Yes, you can just shorten the chain and put it on one chainring and one cog in the back and it would work fine as a single speed.

wow that seemed so simple, when i went to the local bike shop they said i might have some problems like it would come off when i ride.

The only thing you'll have to worry about is whether or not you end up with good chain tension. If not its a simple buy to get a SS chain tentioner.

if I dont put the SS Chain tensioner what would happen with my chain tension??

From the third picture it looks like your frame has vertical dropouts - This makes single speed conversion more difficult, but not impossible. Read about it here:

https://www.sheldonbrown.com/singlespeed.html#tension

You could just put the chain on a cog and go, but a more elegant approach would be to get a bunch of spacers (dismantling old cassettes is a good source) and choose the cog that will give you the best gear ratio. Then you can arrange the spacers to put the cog on the freehub body so that you get a straight chainline.

one of these will give you all the parts you need