Hi,
if you look online there are a lot of free test that you can start practicing. Usually the "training" website makes also some free test available, that can be used also to understand your level before really go into a real training course.
I would suggest, anycase, to consider a trainig course... usually they rpovide a full list of test that you can take in order ot difficultness...
SHL is not difficult, it is really a matter of consistency and a lot of practice. I would start with some free ones, but considering the importance of the challange I would consider also some "on-payment" subscriptions for a month... in order to have the right time to properly prepare... if you do one/ 2 test per day per a month, you can surely train your brain at the right level...
Then it is a matter of properly manage the sress during the official "Test" day
Hope this can help!