That's a nice truck. To be honest, you don't need anything more than an adventurous attitude to go off roading, but that doesn't mean it's a great idea.

I strongly recommend looking locally for some type of offroad clubs. Even a local jeep club will probably be more than happy to have you tag along on some trail rides. Provided you are travelling with responsible people, their experience and tugging should be able to get you off road and through some trails. As you gain experience you can decided where you want to go and what you want to do with your truck.

I recommend going out with others before doing anything to your truck. That being said, your tires won't get you very far, but if you think you are going to lift it down the road, maybe you want to lift it before you buy new tires, but you can get away with a lot of offroading with just a decent set of all terrain tires. Putting in a locker can be less than 500$ or well over 1000$ depending on the type and brand you buy.

There is a lot of different paths you can take so you should have an idea of where you are headed before putting money into your truck, especially if you don't have much.

Keep reading around this forum, especially the 4th Generation section because you have a 4th generation 4runner.