Hi Ibiscus
There are two possible constructions here:
(1) I consider that practising sport is fundamental.........*
(2) I consider practising sport to be fundamental.........*
In (1) you omit 'that', in theory. But I wouldn't recommend it here. On the one hand, 'consider' is quite a formal word, and you keep the 'that' in more formal language. On the other, the sentence starts off confusingly without the 'that': the listener wonders what sports the speaker is considering practising...
In (2) you can omit 'to be', as others have said.
On a point of detail, in British English, the verb is to practise (with an s).
Practice as a noun has a c, though. I believe to practice is correct in AmE.
Loob
* EDIT: thanks to the next two posters for pointing out my typo. I'd love to say it was a deliberate mistake, but it wasn't. I've now changed 'practicising' to 'practising' in (1) & (2). Time to go to sleep, I think...