Ga vs Wa

I noticed when you say I like sushi, it is spoken, o sushi wa suki desu. I have notice on a few other sites people saying just, sushi ga suki desu. Since i am only on Lesson 1.3, i dont understand yet but I read ga is used to emphasize sushi. How do you know when to use wa or ga? also desu, imasu, imasen and masu?

Normally the particle が (ga) is used with 好き (suki) when you say that you "like" something, but it is also acceptable to use は (wa) when you really want to emphasize something. 

When you use は (wa) in this sentence, it's kind of similar to saying "Hey while we are talking about sushi in particular, do you like it?"

Knowing when to use が (ga) or は (wa) will come with time and repeated practice. Just take it on a case by case basis so that you don't get too overwhelmed.

As for your second question on the differences between です (desu), います (imasu), いません (imasen) and ます (masu), they all have a similar meaning of "to be" except that there are slightly different situations when you'll use each one.

I realize that's not a very helpful answer (sorry!), but these words don't exactly have a 1-1 translation into English, so it really is best to learn how to use them within the example sentences.

If anyone else has a useful way of explaining these last four, please jump in! Thanks! 

Desu actually doesn't have meaning on its own other than showing politeness. Tae Kim's work (free online!) goes into detail explaining why. However, for all intents and purposes, it can just be considered a polite way of stating something. 

As for the others, they are the positive and negative forms of the verb iru. Masu itself is just an ending that gives it some politeness. Hopefully this will clear some things up.

Dictionary Form: Iru (Ru - Verb, thus the Ru is dropped when conjugating)

Casual: (Uses dictionary, nai, ta, and katta forms)
Iru - To exist 
Inai (Iru + Nai) - Does not exist 
Ita - (Iru + Ta) - Did exist
Inakatta (Iru + Nai + Katta) - Did not exist
Examples:
Neko ga iru. - There is a cat.
Neko ga inai. - There is not a cat.
Neko ga ita. - There was a cat.
Neko ga inakatta. - There was not a cat.

Polite: (Uses masu forms)
Imasu (iru + masu) - To exist
Imasen (Iru + masen) -  Does not exist
Imashita (Iru + mashita) - Did exist
Imasen deshita (Iru + masen + deshita) - Did not exist
Examples:
Neko ga imasu. - There is a cat.
Neko ga imasen. - There is not a cat.
Neko ga imashita. - There was a cat.
Neko ga imasen deshita. - There was not a cat.

Japanese conjugations can be tricky sometimes!