If you are to marry in Tunisia then once you are wed your wife will have to apply from Tunisia to the British Embassy in Tunisia for a Spouses visa so that she can come to this country and remain her as your wife.
You will have to show:-
1. That you are legally married to each other (ie. by producing your marriage certificate)
2. You are present and settled in the UK (ie. passport, record of address, payslips etc.)
3. You intend to live permanently together here in the UK as husband and wife (statements from you both and others)
4. You have met each other before (Statements as above)
5. You can support each other without the need for public funds (ie. bank statements, savings records)
6. You have suitable accommodation which is owned or lived in only by you or your household and where you and your dependents can live without any help from public funds (mortgage agreement, registered title evidence for property, tenancy agreement
7. That neither of you are younger than 21 years of age (Birth certificates)
There is no automatic right to a spouses visa but provided the above is proved then she should have no problem in receiving one following her application.
You will need to apply for settlement (ie. spouses visa) by using for VAF4 Settlement, available for download from UKBA's Visa website. It would be best for either a solicitor in the UK to prepare it in consultation with both you and your wife before sending it to you so that you can submit it formally to the British embassy.
I wish you all the best with the wedding.
If this is useful please kindly click accept so that I may be rewarded for my time. It will be gratefully received and you will be free to ask follow up questions.