My teacher said something about straining it but I didnt quite get it
The best approach is to ask the teacher again after doing some "homework" (i.e. reading and searching). Extracting and then titrating oxalic acid/oxalate is not that easy from spinach. You will need a lot of spinach to extract a titratable amount. Oxalic acid can be extracted by boiling spinach in 0.1 M HCl. However, tons of other redox active substances will also appear in the extract. Direct titration with KMnO4 will give an extra-ordinary large percentage, which is of course erroneous. After extraction, you need to precipitate oxalic acid via some cation. After isolation of the precipitate you would need to bring back oxalic acid into solution, one can do a redox titration. In real-world nobody would use this approach but as an academic project, it could be a good exercise. Again ask your teacher if she/he is willing to go to this extent.