Add a booster pump after your water tank outlet and all your low pressure solved.

If your pipes are indeed badly clogged, then you would first need to clean up your water tank and ensure that any water coming in are clean of mud, etc. This needs to be address first before you start clearing the clogged pipes. Else it would be an effort in vain.

Clean the water tank and install water filter at the incoming mains.

To clean up the water pipe, just remove the individual stop cock spindle and flush the respective pipe with pressured water. This is to allow the removal of any of the debris in the pipe. If your mains are strong, you could use them. Else, may need a booster pump.

If you are not familiar with these works, better to get a plumber to do it for you.