You can also check this link on XI certification details and from where to start to learn XI:
Also check these starter series of blogs written by Sravya:
I hope this will resolve all ur cocerns. If still you have any query, feel free to ask.
To do XI certification you should have good knowledge of XI first. As you mentioned that you are a ABAP consultant and I suppose you do not have much knowledge of XI till now, so start learning XI first and then go for certification. There are two types of XI certification: Technical Consultant and Development Consultant. Being a developer you will go for Development Certification in XI stream. Below are few links that will guide you for certification need and what are the requisites for XI certifcation:
SAP Consultant Certification Requirements:
Blog on A Beginners Guide to XI Certification Details:
P.S. Reward points if answer is helpful.