Here are some ways to write this differently.

Write about what happened today

First, I overslept :( I hate when this happens, especially because it was a beautiful Friday and I didn't even go(?) outside.
I've spent the whole day sitting on here. I would say it was such a waste of time but let's be positive and forget about it.
I bought/I've just bought a pair of pants which were expensive but I hope they were worth it (is this right? it doesn't sound right). I hate spending a lot of time on the Internet, as it always feels like there's so much more to life waiting for me outside my house, while I'm in front of a big screen harming (harming?) my eyes and buying stuff I don't even need.

Write about what happened today

First, I overslept :( I hate when this happens, especially when because it was is a beautiful Friday as it was today and I didn't even go(?)/venture outside.
I've spent the whole day sitting on here, I would say it was such a/a total waste of time but let's try to be positive and forget about it.
I've bought a pair of pants which was were (even though it is one pair, it is plural) expensive but I hope it was/they were worth it (is this right? it doesn't sound right). I hate spending a lot of time on the Internet, it always feels like there's so much more of life waiting for me outside my house while I'm in front of a big screen hurting/harming/damaging my eyes and buying stuff I don't even need.

Really nothing needs correction, just some minor improvements

Write about what happened today

First, I overslept :( I hate when this happens, especially because it was a beautiful Friday and I didn't even go(?) outside.  (Yes, go outside is right.)
I've spent the whole day sitting on here, (Period instead of a comma.) I would say it was such a waste of time but let's be positive and forget about it.
I've bought a pair of pants which was were expensive but I hope it was worth it (is this right? it doesn't sound right) (Other than the was/were mistake, it is perfect.). I hate spending a lot of time on the Internet, because it always feels like there's so much more of (to life instead of of life) life waiting for me outside my house while I'm in front of a big screen hurting (harming?) (Either hurting or harming works.) my eyes and buying stuff I don't even need.