I have this rashes that tends to scratch me most...

How to diagnose the cause of skin rash and scratching?
There are many causes and those include:
Allergy,
Dermatological disease
Side effects of drugs
Systemic illness
Local infection
 
What tests should you do ?
CBC, ESR, CRP, RF, AST, ALT, creatinin, urea
 all theses tests should be ordered according to dermatologist as needed
 
How to manage skin rash and scratching?
avoid using hot water to shower
shower every other day instead of every day
keep your shower time to less than 10 minutes
use a moisturizing soap when you shower
apply moisturizer immediately after showering
avoid itching or scrubbing dry skin patches
use a humidifier in your home
drink plenty of water
If there is itching and wide space of skin rash in this case you need to use dexamethasone injection and also you need to use topical hydrocortisone 1%
I also recommend to use antihistamine medications like fexofenadin120mg  to easen symptoms.
if you have a systemic disease causing itching you should see an internist
What is my advice to you ?
you need to consult dermatologist to help you and to follow up treatment