You can either run SPMetal to generate static objects to query your lists using LINQ
- http://msdn.microsoft.com/en-us/library/ee538255%28v=office.14%29.aspx
- http://zimmergren.net/technical/sp-2010-getting-started-with-linq-to-sharepoint-in-sharepoint-2010
Or just use an old and yet effective CAML Query
SPQuery oQuery = new SPQuery();
oQuery.Query = "<Where><Eq><FieldRef Name='YourField'/>" +
"<Value Type='Text'>Insert value to search here</Value></Eq></Where>";
SPListItemCollection collListItems = oList.GetItems(oQuery);
In a straight forward linq query
var results = MySPList.Items.Cast<SPListItem>()
.Where(SPItem => SPItem["YourField"] == "Your query");
However, running spmetal is the way to go if you plan to use linq queries frequently, because using straight linq queries on splists can result on serious performance issues.