What openings are recommendable for beginners to study? I am a beginner. With reference to opening theory, which opening should I start learning?

My friends are suggesting Ruy Lopez...
To answer your question directly, your friends aren't wrong, and it's not a bad idea at all for you to play the Ruy Lopez, from both the white side and the black side. In fact, knowing nothing much about you or your exact level of play, it's still a safe suggestion that opening with either 1. e4 or 1. d4 with white, and responding with 1. e4 e5 or 1. d4 d5 with black is a solid way to begin to get your footing in the opening phase of the game. Of course, that's just move one (and only the two most popular options for that), and there are plenty of forks in the road beyond that point. Nevertheless, while my advice to you is that playing the Ruy Lopez is a great idea, studying it (or any other opening) in any great depth is not, at least not right now.

Instead, you should focus on a few principles, like aiming to get your king safely castled in a timely manner, trying to control the center of the board with your pawns (our first moves above are a good start), and developing your pieces with the idea of using them to further control the center of the board. All of these ideas are visible in the Ruy Lopez, and that's one reason it is a good suggestion. After 1. e4 e5, we see that 2. Nf3 develops a kingside piece and helps control the central squares d4 and e5, 2. ... Nc6 fights for control of those same squares (and defends the e5 pawn), and 3. Bb5 indirectly fights for those same squares by threatening the c6 knight, while also clearing the path for the white king to castle. Part of my point is that all of these moves make a great deal of sense, just for the principled reasons given here, and can be recognized as such and played confidently without needing to get your hands dirty with a lot of opening theory.

To learn more about important opening principles, and what you should be thinking about in the opening phase of the game, you might consider checking out John Emms' book Discovering Chess Principles: Building Opening Skills from Basic Principles, which is aimed squarely at developing in beginners a good sense of how to make good moves in the opening. (You'll find good reader reviews at that link; for good measure, here's a review from Chessville as well.) In addition to that, I'd say what's most important is just to play a lot and have fun, keeping guiding principles in mind while you do, and when things do go wrong, make a point of determining why. Try to figure it out yourself, and any time you can't, seek out an explanation from someone else. (Maybe come back here and ask!)