Please try below code :-
 

public class myclass 
{
    @future
    public static void insertUserWithRole() 
    {

        Profile p = [SELECT Id FROM Profile WHERE Name='Contract Manager'];
        UserRole r = [SELECT Id FROM UserRole WHERE Name='COO'];
 
        User uObj = new User();
		uObj.alias = 'ramu';
		uObj.email='prasanthssn@gmail.com'; 
		uObj.emailencodingkey='UTF-8'; 
		uObj.lastname='rampandu';
		uObj.languagelocalekey='en_US';  
		uObj.localesidkey='en_US'; 
		uObj.profileid = p.Id;
		uObj.userroleid = r.Id;
		uObj.timezonesidkey='America/Los_Angeles'; 
		uObj.username='saiprasanth@gmail.com';
        insert uObj;
    }
}

Please check more detail of future method on below blog:-

http://amitsalesforce.blogspot.in/2015/02/future-methods-in-salesforce.html


NOTE :-
1) Methods with the future annotation must be static methods
2) can only return a void type
3) The specified parameters must be primitive data types, arrays of primitive data types, or collections of primitive data types
4) Methods with the future annotation cannot take sObjects or objects as arguments.
5) You can invoke future methods the same way you invoke any other method. However, a future method can’t invoke another future method
6) No more than 50 method calls per Apex invocation
7) Asynchronous calls, such as @future or executeBatch, called in a startTest, stopTest block, do not count against your limits for the number of queued jobs
8) The maximum number of future method invocations per a 24-hour period is 250,000 or the number of user licenses in your organization multiplied by 200, whichever is greater
9) To test methods defined with the future annotation, call the class containing the method in a startTest(), stopTest() code block. All asynchronous calls made after the startTest method are collected by the system. When stopTest is executed, all asynchronous processes are run synchronously

IMP:-
The reason why sObjects can’t be passed as arguments to future methods is because the sObject might change between the time you call the method and the time it executes. In this case, the future method will get the old sObject values and might overwrite them.  To work with sObjects that already exist in the database, pass the sObject ID instead (or collection of IDs) and use the ID to perform a query for the most up-to-date record. The following example shows how to do so with a list of IDs

Please let us know if this will help you