Pozdrav i dobrodošao.

If you don't mind, I will make a few corrections for you.

"Ja Marco jesam."

You mean just "Ja sam Marco". "jesam" does mean "I am" but it is only used at the beginning of sentences, like a one word response, or for special emphasis. While that sentence would be understood, the position of jesam sounds very unnatural to me.

"Učim srpski / srpskohrvatski, ali ne dobro sam."

You might want to instead say "Učim srpski/srpskohrvatski, ali ne (još ) dobro govorim."

The short forms of biti (sam, si, je, smo, ste, su) can follow in the word order with phrases like "dobro sam" but when it is being negated it must always come first - "nisam dobro".

Also, I am dutch should not be "Holandski jesam" but "Holandez sam" (nationality adjects like Holandski aren't used in the same way in Slavic languages as in others. You want to use the noun instead)

It's a hard language, but keep it up and you'll be fine.

Siguran sam da ćeš biti.