Tafsir Ibn Katheer
When starting out on studying Tafseer, I would recommend starting with Tafsir Ibn Katheer. It covers a lot of ground without being too lengthy or too detailed.
Apart from understanding the basic meanings of Quran you will also learn many authentic ahadeeth in this book.
It has been translated in many languages and is easily available everywhere.
The text is copyright free so a free version can be found online.