How can I make sure message m ∈ Zp I am implementing ORUTA and using JPBC library for it.
In one of the algorithms, for hashing a message, it is specified that for message m:  

$m \in \mathbb Z_p$ , for some large prime $p$.  

I have a message text, so how can I make sure that the above equation holds true?
(If possible someone please reply with example code or any predefined funtion able to do this).
It depends on what exactly you want to achieve. I don't know ORUTA.

But in general:
You have to convert your message to a numerical representation somehow. Then the message has to be short enough to be in Zp.

e.g. in Java:

    BigInteger p = [your p value]
    String message = "test message";
    BigInteger m = new BigInteger(1,message.getBytes(StandardCharsets.UTF_8));
    if(m.compareTo(p)&gt;0) {
        not good, your message is too big!
    }


Usually in signature schemes, you use the hash value of the message instead of the message, because a hash function has a fixed output size and for a suitable p you can be certain it is always &lt; p.

This is also quite simple, but if you say you want to compute a hash, it is probably not what you want to do?