Scene 1:
Remember saying simple things like I live somewhere - Dw i'n byw yn... or I work somewhere - Dw i'n gweithio yn... ?
The first part Dw i... is linked with an 'n to the verb. However, if you're using eisiau, you don't use an 'n' as a link.
So, Dw i eisiau mynd, Dw i ddim eisiau coffi. This is something that Welsh learners often forget. Remember: no 'n' before eisiau.

Scene 2:

When you want someone else to do something, you use an 'i', as in Dw i eisiau i chi fynd i'r ysgol.
You notice the letter change here. Mynd (to go) has become fynd. Generally, any verb directly after a noun or pronoun will mutate. Look at these examples:
darllen (to read) - Dw i eisiau i chi ddarllen
gweithio (to work) - Dw i eisiau i chi weithio
cofio (to remember) - Dw i eisiau i chi gofio

We can change chi to the familiar ti, e.g.
mynd i'r ysgol - Dw i eisiau i ti fynd i'r ysgol

Scene 3:
1. Remember eisiau is one of those rare words which is pronounced rather differently to how it's usually spelt. In South Wales, people say ISHE. You might well hear a similar word MOYN, which means exactly the same thing. If someone asks you Beth wyt ti moyn? they want to know what you want!

2. Using a negative. The key word is ddim but you need to change the start of the sentence as well. So a sentence like Mae Siân eisiau car newydd, then changes to:
Dyw Siân ddim eisiau car newydd

3. There are lots of ways of saying yes in Welsh, and the trick is to listen out for the start of the sentence. A sentence beginning with Ydy.... will usually have Ydy as an answer, e.g.
Ydy Siân eisiau car newydd? YDY
Ydy John eisiau mynd adre? YDY
The negative is usually said Na'dy - this is an abbreviation of NAC YDY, which you may also come across. It's ok to say the full version as well.