Tap the Reply button on the message you wish to reply to. (It looks like an arrow pointing to the left.)
Type your reply into the Tweet box as prompted. The box will automatically fill with the @username of the person you want to reply to.
Tap Tweet to send your message.
How do I send a Direct Message on mobile.twitter.com?
Tap on the Me tab.
On the next screen, tap Direct Messages and then tap on the compose message button located at the top right of the screen.
Start typing in the @username of the recipient and select the correct @username from the auto-complete drop-down menu.
Compose your message and when you're ready to send it to the user, tap Send. Your message has been sent!
Note: Direct Messages can only be sent to your followers. If you try to send a Direct Message to a user who does not follow you, you will see an error message. Group Messages are not available on mobile.twitter.com. From your mobile phone, use the Twitter for iPhone or Twitter for Android app to participate in group Messages; from the web, use twitter.com or TweetDeck.
How do I accept follow requests on mobile.twitter.com?
Sign in to mobile.twitter.com on your smartphone's browser. (If you don't have a smartphone, but are accessing mobile.twitter.com through a feature phone or other device, you will have to sign in to twitter.com from a computer in order to access your follower requests).
Under the Me tab, tap Follower requests.
On the next screen you have the option to approve or decline individual requests or to accept all pending requests.
For more details instructions with screenshots, please read this article.
How do I block other users?
Go to the user's profile page.
Tap on the profile icon located under the user's Tweet count.
Select the Block option from the pop-up menu that appears.