Interesting websites to practise grammar: these websites are great because they let you write and then you can check your answers. This is one of the best ways to learn grammar, it's a question of trial and error.
You should begin with those exercises which teach you the grammar form, and then go on to the ones which contrast two tenses, for example present simple and present progressive.
http://usuarios.lycos.es/englishweb/grammar.html (type "mado sanchez" in Google to get here)
REMEMBER TO USE YOUR DICTIONARY AS YOU DO THESE EXERCISES!
Enjoy yourself as you learn! DIY stands for Do It Yourself (Hazlo tú mismo)