Assuming you have cotterless aluminum cranks (based only on the vintage of the bike, since you provided no details). you'd need a square taper bottom bracket for a 68mm, 1.370x24tpi (or British standard) shell. This is the most common type of bottom bracket, and you will find many choices, but probably a low cost cartridge type is best for you. The last variable is the spindle length, which you can measure by taking the cranks off with a crank puller.

Note that changing a bottom bracket can require a number of fairly pricey tools, a situation made worse because the new one may require different tools than the one you're removing. So this is something to consider and it could easity be less costly to let the shop do this job.

Also, before spending a penny on a bottom bracket, make sure that's the real problem. From your description, my first guess would have been a loose/work crank arm. If you've been riding for any length with a loose crank, the crank itself will be damaged, and won't be any tighter on anew bottom bracket than it is now. Regardless of what the real problem turns out to be, making the effort to have a good diagnosis will prevent you from wasting time and money on something that won't solve the problem.