sql - what should i index -

i'm complete index dummie, i've read , watched quite bit of vids on indexing can't seem figure out.

the table looks following:

id          int componentid int value       float timestamp   datetime 

imagine database having 2 million records , following:

select value log  componentid = x  , timestamp >= select convert(varchar(8),dateadd(day, -1, getdate()),112) 

that should me values selected component within last 24 hours

and im wondering if should index on componentid, timestamp or both

thanks in advance!

i suggest create covering index on table like:

create unique nonclustered index idx_indexname on dbo.tablename(componentid, timestamp)  include(value) 

this should improve performance of query index seek. value stored in leafs of index rid of lookups data selecting in index.