Hello and welcome. I'd post a bit more and earn some more money before putting your pet in the arena. It's very expensive getting them healthy again, and some of the pets on here have very impressive stats.
You can buy Strength, defence and agility from the pet shops, but they cost. Get into the forum and make some decent posts. A word of warning, you may be penalised for making daft posts purely for cash. There's plenty of stuff going on in the forum and on the Stanley site that you can join in with. To make a challenge, put your pet in the arena, it won't be too long before somebody joins you.... Have fun
You have to have over 5 posts to use the arcade, and pussycat is right, open a bank account with some of your money, it's a good interest rate. I never fight my pet, but there are plenty who do. Once you've opened a bank account, if you need any dosh to get started let me know, I'm minted...