If you outsource your website you will usually need to hand over your password so they can upload the new website to your server, change the domain name to the new server, etc. It doesn't meant that you will lose control of the website because in the end you will always have control. But first you should make sure of the following:
- Pay a 3rd party company for hosting (instead of hosting your website directly with your web designer) - you should purchase and set up the account yourself, once you have a domain name. In the worst case scenario if your web designer changes the passwords you can always get control back by calling the web host.
- You own your domain name (purchase it from a domain registrar like godaddy.com beforehand). Worst case scenario is the same as above.
First and foremost don't outsource your website to someone or a company you don't trust. Always check several references. If they don't have a good track record, or any previous examples of work, don't work with them. In the end you have to trust your web designer, so pick someone you can trust.
Finally, you can always change you passwords at any point.