mysql create index on table with a 100 million rows

mysql create index on table with a 100 million rows

I have few MySQL tables-these have around 300 columns and 100 million
rows. These store data for log files, hence the size. I am using InnoDB
engine. Few queries involving joins of these tables obviously do not work.
I tried adding indices to these, but the queries do not finish at all.
I wanted to know if there is any other way to speed up performance, or
some way to make the 'create index' work on the tables?
Thank you.