This is my first entry & I don't know what to write about

Hellooo,

Well, I guess you already know that I'm new at Italki and that I didn't find out know what I was going to write about, but I know now: About yesterday.

It's because yesterday was the first day I made cupcakes. I wanted to make cupcakes since the first time I watched on TV "Cupcake Wars" on TV, mostly because it seemed so easy to make. But as in the world not everything is rainbows and puppies; I had a problem: I didn't have a mixer.
So, after thinking a long time, I decided to mix all the ingredients using nothing but a fork, but it took a looooooong time. When I finally finished mixing up all the ingredients, I had another problem came out: I don't have a cupcake tray.
When I thought all my effort was in vane vain, I saw plastic glasses that I could use. So I put the mass mess in each one, and put them in the microwave (because in the hoven they would have melted), and guess what: they melted anyway. I was amazed when I opened the microwave, those plastic glasses where thick and I didn't think the microwave could reach such a high temperature. I put them out of the microwave and put them into the fridge, because I had to hide them from my grandmother, whom bought the glasses (but anyway she does never uses them). So when it was cold enough it I took them out of the fridge and ate the four cupcakes I made, and guess what: They weren't sufficiently baked.
After finishing eating the last one, I felt like my stomach was going to explode. So I layed down and fell sleep until today, waking up with a stomachache.

And that was my first entry, I hope it's OK.
Thanks for reading

Well done!

This is my first entry & I don't know what to write about

Hellooo,

Well, I guess you already know that I'm new at Italki and that I didn't find out didn't know  o wasn't sure what I was going to write about, but I know now: about yesterday.

It's because yesterday was the first day I made cupcakes. I wanted to make cupcakes since the first time I watched "Cupcake Wars" on TV, mostly because they (the cupcakes) seemed so easy to make. But as in the world not everything is rainbows and puppies, I had a problem: I didn't have a mixer.
So, after thinking a long time, I decided to mix all the ingredients using nothing but a fork, but it took a looooooong time. When I finally finished mixing up all the ingredients, another problem came out  appeared: I don't have a cupcake tray.

When I though all my effort was in vain, (o thought all my efforts were in vain) I saw plastic glasses that I could use. So I put the mass dough in each one, and put them in the microwave (because in the oven they would have melted), and guess what: they melted anyway. I was amazed when I opened the microwave, those plastic glasses where thick and I didn't think the microwave could reach such a high temperature. I put took them out of the microwave and put them into the fridge, because I had to hide them from my grandmother, whom bought the glasses (but anyway she does never use never uses them). So when it was cold enough I took them out of the fridge and ate the four cupcakes I made, and guess what: They weren't sufficiently baked.
After finishing eating the last one, I felt like my stomach was going to explode. So I layed down and fell sleep until today, waking up with a stomachache.

And that was my first entry, I hope it's OK.
Thanks for reading

Great job!

Lay vs. Lie 
Infinitive   Definition           Present      Past       Past Participle   Present Participle

to lay      to put or place      lay(s)        laid        laid                  laying
             something down

to lie to rest or recline         lie(s)         lay        lain                  lying

I rested or reclined (en el pasado) = I lay down.

"I mixed up all the ingredients"is ambiguous in the context of your story because to mix up could mean mezclar o confundir.  Better just to say "I mixed all the ingredients" because "mixed" is simply mezclar.