18 speed mountain bike problems?

18 speed meaning what?  Is it a 2 X 9 drive-train or a 3 X 6?  Is this your bike? https://www.walmart.com/ip/Hyper-26-Shocker-Men-s-...  What do you expect from Wally World (Walmart) for $124.00?  Discount store bikes are also known as BSO - Bicycle Shaped Objects.

Wally World does NOT service what they sell.  They can't even assemble them correctly.  So now you have maybe 3 choices.  A) If it's still with the exchange / return policy - TAKE IT BACK FOR A REFUND.  B) Take it to a REAL bicycle shop for a tune-up.  They might find more wrong with the bike.  C) Learn to do it yourself.  Start with this web link from Park Tool.   https://www.parktool.com/blog/repair-help  

Rear derailleur troubleshooting -  https://www.parktool.com/blog/repair-help/rear-der...  

The brake pads should be slightly toed-in.  That is...the front part of the pad should touch the rim first.  This is also covered in the Park Tool link.  When will people learn a cheap bike cost MORE in the long run?  You want a good bike with a local service warranty?  Go To A REAL Bicycle Shop    

There are plenty of youtube videos showing how to adjust derailleurs. Until you go though them, you won't know if your problems are adjustment-related or if you just have crappy derailleurs that won't work right. It's a given that nobody at the department store knew how to adjust them.

maybe you can get a mechanic to fix it