If you are new to learning programming and do not know where to start, please try Problem A “Welcome to AtCoder” from practice contest. There you can find a sample code for each language.
Also, if you are not familiar with problems in programming contests, we recommend you to try some problems in “AtCoder Beginners Selection” (https://atcoder.jp/contests/abs).
「競プロ典型 90 問」(Typical 90 Problems of Competitive Programming) (https://atcoder.jp/contests/typical90) is a collection of typical 90 competitive programming problems; unfortunately, currently the problem statements are all Japanese.
「C++入門 AtCoder Programming Guide for beginners (APG4b)」(https://atcoder.jp/contests/APG4b) is a C++ tutorial for competitive programmers. Sadly, this is only in Japanese too.

The length of a string can be obtained by s.size() in C++ or len(s) in Python. Note that many languages adopts 0-based indices.

Sample code (C++):

#include<bits/stdc++.h>
using namespace std;

int main(){
    string s;
    cin >> s;
    cout << s[(s.size() + 1) / 2 - 1] << endl;
}

Sample code (python):

s = input()
print(s[(len(s) + 1) // 2 - 1])