If you search the internet on how to export Symbian contacts in .vcf format, you'll find a lot of hits on how to do that. Once you export them, what you do next depends on what you want to do with the contacts, besides get them onto the iPhone. If you have a Gmail account, you can use your computer to import the .vcf file export from your Nokia, then set up your Gmail account on your iPhone and turn on Contacts, they'll start syncing automatically.
If you don't use Gmail then perhaps another email account type could accomplish the same thing. However, iCloud cannot read .vcf, only .vcard, so unless Symbian can export to .vcard then there's no direct way to get them into iCloud if that's what you prefer.
Another option is to use a 3rd party app for your iPhone to (hopefully) import a .vcf file, though there could be a lot of variables there that I'm not aware of.
Bottom line though, what you don't want is to have your contacts only living on your iPhone, you want them hosted by some web service (Gmail, Yahoo, iCloud, etc.) and then sync them to/from the account. That way, if you lose your phone or have to erase it, you can recover your contacts simply by re-establishing the account syncing to your phone. If you're worried about keeping your contacts on the internet, then find a computer-based contacts manager to host your contacts and sync them to your iPhone through iTunes.