2 ways. Register by yourself, or hire some company services.
I use the following company to file my tax, company is nice
If you want to register yourself, you need to register on NTN and SECP.
for tax registration https://iris.fbr.gov.pk/public/txplogin.xhtml
for SECP registration guide https://www.secp.gov.pk/company-formation/registration-of-company/
SECP also makes a Startup portal for startups in Pakistan
here you can find videos and other stuff to start a company.
I suggest, trying an easy way get to hire some firm or individual lawyer for legal work.Here is one firm I found by google search https://emerhub.com/.