Many of my Thai friends asked me, how have you learn Thai so quickly? 
I tell them that I practise every single today. I consider myself to be in between the B2 and C1 levels in terms of my Thai language proficiency.  Ive been learning for about 3 years and live in UK and am hardly in contact with any thai people (I have never lived in Thailand).  Again, many Thai friends ask me how can they learn English quickly. basically learn by listening>speaking>reading>writing.  Just the same as a baby when they are learning their mother tongue language.  Some friends say my accent is quite close to a native accent (this varies depending on where my thai language partner lives in thailand).  
Unfortunately, at the start u need to learn lots of basic vocabulary. For example, i started by learning vocabulary through thai songs (i listened to clash's songs). I started learning basic words like เธอ, เรา, รัก, ฉัน, เสียใจ, คิดถึง... After you learn new vocabulary, you should try to write them down and review them a couple of times a day. After you've learnt around 200 basic words, you should try and find a language partner in an app such as Hellotalk, italki, tandem or even interpals (warning you may meet perverts!).  I recommend speaking with many people so that you get used to the different accents of native speakers. For Thai people, I found that you are more used to the american accents than British ones.  In this case, you should focus on speaking with British people or watch British TV programmes. 
Since you're learning english youll probably be lucky enough to speak english only.  my journey was more difficult. thai people just wanted to speak english so it was hard for me the practise speaking thai at first. Therefore, if u meet a language partner who wants to learn Thai, please share your time. If u dont share your time equally for each language, your language partner will disappear! u have been warned haha. 
I watched ALOT of thai series and dramas. i feel this helped me both expand my vocabulary and improve my accent as well as speak thai more naturally (in terms of sentence structure). I watched thai series such as Ugly Duckling, hormones, room alone, club friday, U prince...
I watched all of these WITHOUT english subs, even at the start. you should try watching american or british series (walking dead, breaking bad, teen wolf, the flash, sherlock...).  Watching TV without the subs means that you are focusing on listening and not just reading the subs.  Of course, you need to have a good vocab base before you start watching TV programmes.
Reading can help too. Dont just read educational textbooks such as TOEIC  but try to read something you ENJOY. for example, in highschool I loved reading manga. so i bought some manga written in thai. I also have some แจ่มใส books... haha. I am also reading Cancel my Cancer, which is an emotional autobiography about a Thai girl's battle with cancer.  Reading books is the best way to learn new words. However, you must keep a vocab list and try learning the new words. 
Hope this helps.