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.

One can convert a lowercase English character \(c\) to uppercase with, for example in C++, toupper function:

// Let C be uppercased c
char C = toupper(c);

Thus, it is sufficient to uppercase each character of \(S\) with a for statement.

Sample code (C++)

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

int main() {
    
	string S;
	cin>>S;
	
	string T = "";
	
	for(int i=0;i<S.size();i++)T += toupper(S[i]);
	
	cout<<T<<endl;
	
	return 0;
}