Traveling from Warsaw to Grajewo isn't a problem, you can manage it easily without any knowledge of Polish
Regarding train schedules there is a few direct trains per day and a couple of options involving one change (in Białystok):
As you can see trains departing 10:55am & 17:22 are direct ones, while with departures 7:30am & 13:55 short change in Białystok will be required, however you can purchase one through ticket, so you won't have hassle with buying two different ones
Return journey timetable from Grajewo to Warsaw as follows:
What concerns tickets, then fares are 49,9 PLN for direct IR train or 56 PLN for direct TLK train (and for both direct trains you can actually purchase tickets online via official PKP PR and PKP IC websites), while with changing trains REG+TLK options will cost you 65,6 PLN or 43,1 PLN when using REG+IR trains. If you plan to purchase your tickets at Warsaw station, then it might be a good idea simply to write down the route, train numbers (or schedule) and travel date you need on a piece of paper and hand it to the cashier, this way you'll avoid any possible misunderstanding