Christian Mingle, E-Harmony, Match and CDFF are probably the best sites, in my opinion. Plenty of Fish is overrun by scammers and catfish, although they will be present to some extent on any dating site, even paid sites.
In terms of tips, I would recommend not to take it personally if someone doesn't respond, or if she drops communication after a while. She may just be shy, feel uncomfortable or not be sure how to reply. Focus on creating a positive, enjoyable and comfortable environment for her to reply, and leave the results up to God. Don't treat it like a question-answer session basically (which I'm guilty of in the past lol), and if the conversation does stall, don't fret - it can still be revived.
Some members online are also incredibly shallow and self-centered, even on Christian dating sites. They're just looking for fun, not something serious. I see it as a form of protection from God when He doesn't bless a conversation, and recognize His intervention in the midst of it.
Also, make sure you establish clear intent and direction in the conversation early on, perhaps after you break the ice. If you're seeking a companion in contrast to friendship, make sure the person you're getting to know truly has a heart for God, and is good relationship or marriage material. She doesn't have to be perfect. But, if she's at least willing to adapt to some extent, communicate and work things out, a relationship is possible.
Hope the search is going well.