Hi Carol. There are a lot of factors involved, so we need more information to give you a better answer. As others have said, it is always best to test the oldest generation first. However, you also have to decide which company to test with and what type of test. You will find the most relatives by doing an autosomal test. Ancestrydna has the largest database. From Ancestry, you can upload to familytreedna, MyHeritage, and gedmatch. Uploads are free, but there are small fees for added services. You can also do autosomal testing at 23andMe if you want to look for more matches. Neither Ancestry nor 23andMe accept uploads, so you have to test directly on those sites. If you are interested in Y-DNA, you have to test directly on ftdna (family tree) since an uploaded kit can't be used for that. Or you can test one of the men at ftdna with family finder (autosomal) and do the Y-DNA test later using the same sample.
You don't say who you are in the family, but if you are a sister of the two brothers, your DNA will also be helpful. My suggestion is to start with the father and one of his children at Ancestry. If you all have the same mother (if she is deceased), you will be able to separate paternal and maternal matches. Then upload to the other sites. If you are interested in doing the Y-DNA test, one of the brothers should do the autosomal test first at ftdna so you can compare to the other uploaded kits. As I said, you can do the Y-DNA test later, but there should be no rush on that. If there are any maternal living relatives (aunts, uncles, or even cousins), you might want to consider asking them to test, also.
It can get very expensive, complicated, and time consuming but also thrilling! Good luck!
Researching maternal surnames:GOLDBERG, POLAK, SEGALOVICH, LISNIK (All from Minsk)
and Paternal surnames: MEREMINSKY, WILKOFSKY, POSCIVE (from Peski and Ostrow)