Should I trademark nationally or internationally if I will be taking clients worldwide?

Trademarks are intangible assets, and can be bought, sold or licensed, making your company more valuable. Trademark establishes a link between enterprise and customer.  A strong trademark will attract customers to use goods or services.

Do you have to buy trademark in all countries? 

What's the best way to register an international trademark?

However, you have to analyze your company to determine the value in seeking trademark protection in foreign countries. An experienced intellectual property (IP) attorney is the best person to speak to regarding this decision. There is a mechanism for obtaining a trademark in other countries: the  Madrid Protocol

 International Trademark Registration Under Madrid Protocol/ Agreement

To sum it up, you should first file for a trademark in your home country. From there, your country will forward it to WIPO. After WIPO examines it, and hopefully approve it, it will send you a certificate of your international registration and notify the IP Offices in all the territories where you seek trademark protection in. The last step is for those territories to make a decision within the time limit (12 or 18 months) in accordance with their own rules. WIPO will record the decisions of the IP Offices in the International Register and notify you when they are made. If a particular Office refuses to grant protection, you can contest a refusal decision directly before it. Conversely, if an IP Office accepts to protect your mark, it will issue a statement of grant of protection.