Hi!
So, long story short, i have a young tree that i want to turn into an bonsai. I thought it would be the most fun to plant them myself and watch them grow from sapling to mature tree. The only problem is that i collected these seeds in nature, so i don't know the name of the species...

I always wanted a bonsai, it would be really helpful if someone can tell me the name of this plant! (yes i searched on google but i can't find anything, i think it is some maple but i am not sure)

It doesn't look like a maple to me but I don't have another suggestion for you. Its a little young to start pruning and shaping for bonsai. Give it a couple years.

Ivap said:... i collected these seeds in nature, so i don't know the name of the species...

It would help to have some idea of where the seeds were collected - city/state/country, etc.

It's a basswood (linden) species. Whatever mature trees grow nearby, that's probably the one.

Thank you for your information!
The seeds were collected in south-east europe. I don't think it is basswood because the leaf shape doesn't match...(maybe it will change to turn into basswood like in the pictures?).

Basswood in North America is the common name for Tilia, of which there are several different species/hybrids in Europe. The initial seedling leaves look like yours but the later ones get the more mature and typical shape. Here are some pictures of Tilia seedlings to compare with yours:

https://www.google.ca/search?q...

This one is a European species, Tilia cordata:
https://www.google.ca/search?q...:

Ivap,
Your photos are showing the seed leaves aka cotyledons which are much different than the true leaves. Look on this site for a good photo of Basswood/Tilia cotyledons.
http://www.be-roberts.com/se/h...

Tilia cordata (Little leaf Linden) is a common tree in many parts of Europe, including the southeast. In Slovenia, it is a national emblem. Many Slovenians, including my relatives there, still harvest the linden's opening flower buds to brew tea.