How much should i put in my 401k monthly/yearly?

As much as you can up to the $18,000 limit ($24,000 if you're over age 50)! But if that seems like a crazy amount, then at least make sure you're contributing enough to receive any match your employer may offer. For example, if your employer offers a 6% match, then you should put in at least 6% to get that "free money."

If you don't have a match or you want to contribute more, 10% is a good rule of thumb if you're in the earlier years of your career. If you're getting a late start, bump that number up to 15%. Check to see if your 401k plan has an auto-escalator tool where you can set the amount you contribute to increase on an annual basis by a percentage point - this makes it a little less painful to increase your contributions and makes it automatic so you don't have to remember to do it.

You can try using a retirement calculator to estimate how much you would need to save. Here is one, for example: 

https://money.cnn.com/calculator/retirement/retirement-need/

The calculator, as well as other rules of thumbs, however, only provides a general guideline. Each person may have a different retirement goal, risk tolerance, savings, debt, etc. Generally you can try to contribute as much as you can and as early as you can. 

Another thing to consider is to set up and contribute to an IRA account, so that you can have access to more investment options. You can take advantage of any employer matching to your 401k, then contribute to an IRA instead.