If you're having problems with low water pressure and your toilet, there are a number of ways to increase toilet water pressure that you can try. In most cases, low water pressure is caused by increased demand for water (a temporary issue) or by a faulty part. Regardless of the cause, increasing toilet water pressure can be tackled by the average person with these steps.
Check to make sure you're not using too many water-consuming items at the same time. If you're flushing the toilet at the same time you're running sprinklers, someone is taking a shower, and the washing machine is running, you don't have a water pressure problem--you have a very temporary water pressure issue caused only by excessive demand.
Check the toilet valve (normally located under the tank and next to the bowl of the toilet) and make sure it is completely open. Typically, this means turning it clockwise until it is fully open.
Check the fill valve on the toilet. If it has failed, you may need to replace it.
You may need to increase the whole-house water pressure. Your pressure regulator can be found below the front hose connection in your home, and is designed to set the water pressure for the whole house. Check to see if this is still functioning. If it is, increase the pressure.
If the pressure regulator is broken, replace it. Call a plumbing professional for this step.