It has been said that one of the best ways to learn a language is to get a boyfriend or girlfriend from that country who speaks that language you want to learn, as in the best way to learn a language is horizontal. While that may be true, it is not the only thing you can do to help you grasp the rudiments of your chosen language. Total immersion in the language, learning words with some memory tricks, getting a practice partner, keeping a notebook for writing new words, setting yourself tasks of learning a certain number of words per day, finding a class with a good teacher and a good book to work with. All of these things will help you in your quest to be fluent.
A good teacher is important in a face to face classroom type of learning, you should get one that will explain things slowly and only speak that language that you are supposed to be learning. There are free Spanish lessons online which will help, but talking to real people directly is what it is all about in the long run. Online Spanish courses are good for things like flash tests that will help with vocabulary and often they have audio with them too, so that you can hear the word from a native speaker.
What is the hardest language to learn? Well it will be more difficult when you have a totally different alphabet to learn and even more difficult with something like Arabic, Chinese, or Japanese where the writing doesn't look like an alphabet as we know it, more like symbols than letters. But then learning a language like that can be done with more of an emphasis on the spoken to start with and concentrate on the hearing, understanding the sounds and getting to be understood when you say things yourself while out and about in that country.
What would be the easiest language to learn or the best language to learn? The easiest I would say are the languages that are phonetic, in that once you have learned the alphabet sounds then you can put the words together. Spanish is a language like that and so is German. If you see the word on the page then you can say it.
A memory trick is to make pictures in your head to remind you of a word. For example the Spanish word "esquina" means corner. To help you remember that word you might envision an image of a pair of skis in a corner of a room. The word for fork in Spanish is "tenedor" so imagine ten forks sticking in a door. Simple tricks like that do help with your Spanish language lessons. That will work with whatever language you decide to have a go at. One of the ways you will get a new language in your head is to hear it around you, even if you don't understand all of it. It helps you to get the flow of the words right so that you sound less of a foreigner when it is your turn to have a conversation. So listen to radio and television in your chosen language.
The author has used these techniques. Mother tongue of English, remembers some French from school, many years ago. Speaks good German and Spanish and understands quite a lot of Catalan, which is necessary living in Catalonia.