It has to mach the noun's case - in this sentence poduszka is in Accusative = ta (this) also has to be in Accusative. Tą is Instrumental case - you would say: Ona jest tą dziewczynką (She is this girl).
As you can see in both examples endings matches - -ę -ę and -ą -ą. In different cases it is harder: Nie widzę tej dziewczynki ( I don't see this girl) = endings don't match.
But the rule of thumb is: ą/ę ending has to match. EDIT: this is true only when using pronoun 'ta'. When you use adjectives Accustive and Instrumental have both-ą ending:
Acc: Mam miękką poduszkę. ( I hve a soft pillow).
Ins: Śpię z miękką poduszką. (I sleep with a soft pillow).
When you think about it, the above example is probably the cause of inapropriate use of 'tą' - everywhere else the ending is -ą, but not in this instance.
It's better to say 'tę poduszkę'. The form 'tą poduszkę' is acceptable in casual speech, but not in writing or official speech.
when talking about anything but human beings you may find attractive:
"podoba mi się" - is initial impression, while "lubię" is more informed, stable affection.
"Podoba mi się ta sukienka" - I say when I first see it in the shop, and later when I choose it , and when I show it to my friend/mom/boyfriend.
"Lubię tę sukienkę" - I say about one of my favourite dresses I feel good in, and often wear.
Now when we talk about humans - podobasz mi się usually means "I think you are attractive". that may not always be true, if you just displayed some admirable aspect of character, somebody may say it meaning they like your character.
Side note – while „witryna [internetowa]” is OK, usually we use „strona [internetowa]” on a daily basis – „witryna” feels like formal register to me, I would not use it outside of business offer(for example). When speaking about Duolingo, I would simply say:
- Podoba mi się ta strona.
And add „internetowa”, only if context would require clarification(for example, if I was talking about books earlier and „strona” alone could be confusing).