Make sure that you have registered to vote
In order to vote in May's local election, you need to have registered. The deadline for doing this is midnight on Thursday 13th April, so you still have a few days in which to do so. If you haven't registered yet, or if you want to change some aspect of your registration, you can do this at https://www.gov.uk/register-to-vote.
Once you are registered to vote, you can also apply for a postal vote (as an alternative to voting in person at a polling station). Postal ballot papers are generally sent out about a week and a half before the date of the election. If you want to vote by post, then the deadline for applying is 17:00 on Tuesday 18th April. The application form is available from https://www.yourvotematters.co.uk/how-do-i-vote/voting-by-post.