Monthly payments can be somewhat difficult to estimate as there are a lot of variables such as, what your credit score is, how many months you want to finance, how much money you want to put down, etc.
A good rule of thumb is that for every $1,000 it is roughly $20 a month. Or to simplify it, for every $5,000 it is roughly $100 a month. In other words, if you have a monthly budget of $300, don't be looking at the $25,000 car as that is roughly $500 a month.
If you want to get a bit more precise, then you can get on an auto loan calculator to put in the different variables. If you do this remember that the national average rate (all the people with horrible credit and excellent credit smashed together) for a pre-owned car is roughly 10.25% while a new car is roughly 7.5%. If you have excellent credit then don't let these numbers scare you as they are an average and not the actual rate you would get.
Unfortunately there is no single loan calculator that I have found that properly covers all the different variables. So I have attached links to three different calculators so you can get a rough idea what type of monthly payment you will be looking at.