It is difficult to judge exactly how badly an IVA will affect your credit rating as it depends on many factors, including what your credit rating is currently like.
Once accepted for an IVA, your details will be placed on the Individual Insolvency Register (IIR) until the IVA has finished. Being on the IIR will negatively affect your credit rating, and will therefore make it more difficult to obtain further credit during this period.
However, the fact that you have been accepted for an IVA quite often means that your credit rating is already quite bad, and if you are struggling to meet payments already, further credit is not a good idea.