How will I know where to park to enter through Centerfield gate?

You'll save a lot of money parking at a farther lot and walking the half mile. It's not bad on a nice day with a lot of walk up attendance.

There is a parking lot by the left field gate but I think its very expensive

Bring oil, wheat, and jewels. You will find your way.

Walking from a further lot or taking the Metro is the way to go. If you park close, plan to pay a TON of money and then sit in the parking lot for 30-45 minutes waiting for the crowds of walkers to clear off the streets before you can think about moving.

Anything near Half St. SE will work.

The price will dramatically change that's how you will know..lol Very high