I agree with Mike Scott's comment that it's best to use something which is already open-source which has gone through all the research and so on. If your doing the project for learning and personal usage then start off with basics such as:
- If you support clipboard don't forget to clear it during application been closed or after a set time period.
- Don't leave text in memory after it's usage
- Do good amount of research into encryption for your database and enforce a good password policy
- Support keyfiles for higher level of security
- Don't store any confidential information as plain text on disk
- Don't support any network related APIs or if you do make sure you validate all your buffers to avoid any remote buffer overflows.
You could go alot more into malware targeting your software using APIs such as FindWindow to read the strings out of your application and code injection using CreateRemoteThread but I'm not too sure of how complexity you want to secure the password from been stolen via malware.
I'd recommend you look into the code of an reputable password manager which is open-source as they will of done the above and more.
I know this list doesn't cover everything but I hope it's a good starting point for you.