The easy bit first.
To get a Scottish birth/marriage/death, census, old church record, will or various other documents, go to
www.scotlandspeople.gov.uk, register and invest in a few credits. Searching the indexes is free but you need to use credits to view the document. Once you have viewed it, you can download it or just leave it on your account for future reference.
This is when it gets more difficult in your case, however. Statutory registration started in Scotland in 1855, so there is no birth certificate as such. You need to look in the church records for anything before 1855. The bad news is that I have done this and failed to find any record of your Agnes Clark's baptism, or of any siblings.
There is a very small chance that it's lurking somewhere in a register that hasn't been digitised, but the overwhelming likelihood is that the record of her baptism, if it ever existed, has not survived.
There is a record of the marriage of Alexander Clark and Catherine Graham in Eastwood in 1829.
You should, however, be able to find her in the 1851 and, depending on when she was really born, 1841 census. Best place to look first is FreeCEN
https://freecen1.freecen.org.uk/cgi/search.plThere is a listing in the 1851 census of Agness Clark, 10 and Archiball Clark, 7, pauper children, in the household of Ann Dowd, pirn winder, in Eastwood (Pollokshaws is in the parish of Eastwood) who might be yours. If so there could be something in the Eastwood Kirk Session records about them, and also in the Eastwood Parochial Board records, assuming that both sets of records have survived.
The Eastwood Kirk Session minutes are in the National Records of Scotland, though there has been talk for a long time about them becoming available on Scotland's People.
For the Parochial Board records I'd start with the Mitchell Library in Glasgow
https://www.glasgowfamilyhistory.org.uk/ExploreRecords/Pages/Poor-Law.aspx