Instruction

First of all let's add function where we are going to validate Fields, and declare it in reduxForm:

function validate() {

}

export default reduxForm({
    validate,
    form: 'PostsNewForm'
})(PostsNew);

Also, we should add a variable as parameter to this function, which will contain all values, which are passed to forms:

function validate(values) {

}

Now we should add a variable, which will contain errors:

function validate(values) {
    const errors = {};
    // Validation

    //IF errors is empty, the form is fine to submit
    //IF errors has *any* properties, redux form assumes form is invalid
    return errors;
}

Finally, validation function could look like this:

function validate(values) {
    const errors = {};
    // Validation
    if (!values.title || values.title.length < 3) {
        errors.title = "Enter a title, that is at least 3 characters!!";
    }

    if (!values.categories) {
        errors.categories = 'Enter some categories';
    }

    if (!values.content) {
        errors.content = 'Enter some content please';
    }

    //IF errors is empty, the form is fine to submit
    //IF errors has *any* properties, redux form assumes form is invalid
    return errors;
}

Final Code

import React, {Component} from 'react';
import {Field, reduxForm} from 'redux-form'

class PostsNew extends Component {

    renderField(field) {
        return (
            <div className="form-group">
                <label>{field.label}</label>
                <input
                    className="form-control"
                    type="text"
                    {...field.input}
                />
            </div>
        );
    };

    render() {
        return (
            <form>
                <Field
                    label="Title"
                    name="title"
                    component={this.renderField}
                />
                <Field
                    label="Categories"
                    name="categories"
                    component={this.renderField}
                />
                <Field
                    label="Post Content"
                    name="content"
                    component={this.renderField}
                />
            </form>
        );
    }
}


function validate(values) {
    const errors = {};
    // Validation
    if (!values.title || values.title.length < 3) {
        errors.title = "Enter a title, that is at least 3 characters!!";
    }

    if (!values.categories) {
        errors.categories = 'Enter some categories';
    }

    if (!values.content) {
        errors.content = 'Enter some content please';
    }

    //IF errors is empty, the form is fine to submit
    //IF errors has *any* properties, redux form assumes form is invalid
    return errors;
}

export default reduxForm({
    validate,
    form: 'PostsNewForm'
})(PostsNew);