I want to make sushi but I'm not going to buy any more rice then I already have, which is brown. my first couple experiments didn't work. What should I do to roll sushi using brown rice?
Normal brown rice just isn't sticky enough (not enough available starch) . Risotto rice might work if you have some anyway, though the texture will be a little different. Apart from the fact that rice keeps forever, if you've got sushi rice to use up, it makes a perfectly acceptable risotto, or fried rice balls. It doesn't need to go to waste.
Where you're going wrong is thinking that rice is rice. It isn't. There are several major categories of rice which behave differently in cooking. Trying to substitute one variety for another, even though they're the same species, is like trying to substitute sweet peppers and chillies.
I have made sushi with brown rice before and can say two things about it: Mine turned out fine for rolls to my tastes, and there is no way you will ever get sushi to turn out "correct" or close to it with brown rice.
Different rices have different characteristics, and you cannot will them to behave the same. Brown is a long grain rice with less polishing. Sushi rice is a highly polished short grain rice that should be sticky enough to hold together, but separate easily to bite. You can get brown rice sticky by cooking a bit longer and with extra water, but you will not fully match the texture for rolling, and it will be very different for hand shaped applications. Further, the taste will never match.
My personal taste what that I like rolls made with brown rice at least with some items that the nutty flavor complemented. I did this with a rice maker with adding about 15% more water than recommended then mixing with a rice vinegar and sugar and salt, mixing and fanning heavily. A bit heavier on the vinegar than with sushi rice. The extra water helps it turn out a little sticky without overcooking. Again, most would not really find is especially nice for hand shaped, but personal taste you may like it for that as well. I did not find it acceptable texture for that myself.