am a beginner ..so can someone suggest me good exchange to buy ERC20?
Exchanges are your best option as long as the ERC20 token you are looking for is listed there. If you are looking for a good exchange, you may just try bittrex since that seems more like a best bet so far. There are some wallets like exodus that lists some tokens too, though might not be much but you can do a swap with your other coins directly from the wallet.
Bittrex is the kind and most trusted site from where you can buy ERC20 tokens. Etherdelta is very complex for a beginner. So bittrex, hitbtc or cryptopia can be best choice for you to purchase ERC20 tokens.
Etherdelta is really complex except for someone who has been using them for a very long time though and even at that it is not user friendly at all. The only good thing is that mostly all ERC20 tokens are listed on this exchange which is a good one, and you can even get them quicker and better.