(Some of the corrections I made are more so for making you sound more fluent and coherent rather than they were wrong grammatically)

Studying (a) language (This makes it sound like you study linguistics instead of one specific language. The "a" will change it to meaning when one studies a language.)

What would be the most important aspect to focus on in order to improve my English? I think the best way to learn a language is speaking a lot. When you speak a lot, the muscles in your mouth will memorize how to articulate the sounds and pronounce the words in that language.

But I think we need patience before we get to a point where we can speak well. In the beginning, we won't be able to find the words as quickly as we would like, but by speaking often in that language our skills will get better and better. (I took out "I think" because you already said I think at the beginning of this thought. There's no need to say it in this sentence as well.)

And it's also very important to speak in a relaxed manner in the beginning. We need to think about what we want to say and how we want to say it. (I took out probably because, just like "I think", it's not really needed. You're stating a fact, but then you're putting "probably" which detracts from the validity of your statement/makes you sound unsure. It's your fact that you're presenting. You should be sure about whether or not it's true, not just "probably" sure. If that makes sense. I'm not saying it to judge, but if I were to hear even a native speaker say it like that, that's exactly what would be going through my head.)

I wrote this for myself today haha. I want to believe that all my practice has improved my English (somewhat). (The "somewhat" at the end is for if you want to put it and downplay your progress, as I sense a lot in your little passage that you do. Downplaying your skills is totally fine, but your English is good, nonetheless.)