My problem when i start my 90 riviera it hard to start

Either it's time for a major tune-up, or your fuel system is losing pressure.
To test this, when first starting the car in the morning, turn the key off and on to the count of ON (five seconds) OFF (five seconds) three or four times, then turn the key to ON and leave it for five seconds or more. Then, start the car and see if it is still difficult to start. If it starts more easily, the fuel system is losing pressure overnight.
Verify that at least 35 psi is retained at the fuel rail overnight. If the pressure is less than 35 psi in the morning after the vehicle as sat overnight, replace the fuel pressure regulator and check for fuel leaks from the rail or supply lines. Check the fuel pump to be sure it is properly operating and that the check valve is not allowing fuel to bleed back into the tank.
If your fuel pressure retention at the fuel rail is good, it's just time for a major tune up.
If you've been using ethanol blend fuel in that vehicle, you may have a moisture contamination issue with the fuel, and you'll need to replace ALL FUEL in the vehicle as a diagnostic measure. Try to avoid using ethanol blend fuels in that vehicle if at all possible.

Sounds to me like the security system is starting to fail and its very common in GM vehicles. I actually had it happen to mine. I did a lot if research and found a company that sells a module to bypass it and allow the car to start every time. I did mine almost 5 years ago and my car still runs great to this day. you can do it yourself and they guarantee the part.. if you have questions they have great customer service.. I hope this helps ya out Do It Yourself GM Security bypass