how to get process handle from process id?

OpenProcess(PROCESS_ALL_ACCESS, TRUE, procId);

You'll need to verify that you're using a valid process ID, and that you're permitted the access rights you request from the process.

Is this what you are looking for?

HANDLE processHandle = OpenProcess(PROCESS_ALL_ACCESS, FALSE, processId);
CloseHandle(processHandle); 

Also, here is some code I use to set debug privledge before injecting DLLs.

void Loader::EnableDebugPriv(void)
{
    HANDLE              hToken;
    LUID                SeDebugNameValue;
    TOKEN_PRIVILEGES    TokenPrivileges;

    if(OpenProcessToken(GetCurrentProcess(), TOKEN_ADJUST_PRIVILEGES | TOKEN_QUERY, &hToken))
    {
        if(LookupPrivilegeValue(NULL, SE_DEBUG_NAME, &SeDebugNameValue))
        {
            TokenPrivileges.PrivilegeCount              = 1;
            TokenPrivileges.Privileges[0].Luid          = SeDebugNameValue;
            TokenPrivileges.Privileges[0].Attributes    = SE_PRIVILEGE_ENABLED;

            if(AdjustTokenPrivileges(hToken, FALSE, &TokenPrivileges, sizeof(TOKEN_PRIVILEGES), NULL, NULL))
            {
                CloseHandle(hToken);
            }
            else
            {
                CloseHandle(hToken);
                throw std::exception("Couldn't adjust token privileges!");              
            }
        }
        else
        {
            CloseHandle(hToken);
            throw std::exception("Couldn't look up privilege value!");
        }
    }
    else
    {
        throw std::exception("Couldn't open process token!");
    }
}

I've used the above code on Windows Vista with success.

You would need elevated privileges. Also look at similar question here.

If you have a process identifier, you can get the process handle by calling the OpenProcess function. OpenProcess enables you to specify the handle's access rights and whether it can be inherited.

FYI:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms684868(v=vs.85).aspx