How to convert a old Wordpress theme to responsive theme

I got some work regarding fixing a old Wordpress site. Now here this site is quiet old and has a simple layout. There is a huge slider on the center with three tabs at bottom. It is mostly based on a company profile. There is nothing else. Now when I open the site in a mobile phone it takes a very long time to open. It is also not fitting well on mobile and tablet resolution. I want some help to make this theme more responsive. Something that can make it very well on mobile platform. Does I have to replace the entire theme.

No there is no need to replace the entire theme. The good part of using Wordpress it hat you get ample of themes in the market which will allow you to get a responsive theme as per your need. There are two things you can do. If you go with a new response theme then you will get a lot of new stuffs. And if you go with a plugins then you can use your existing theme on the mobile phone resolution. If you go with manual css changes then you will need to take help from a developer. Better try a plugin like Responsive Plugin.

I know the best plugin here that can help you to get what you are looking for. You can try using Jetpack. Great plugin to make majority of changes on your site and making it very effective for usage. Just add this in your WordPress site and start using the same. Or if you need something very straightforward then you can also try using WPTouch Mobile Plugin. It is also an amazing option for have a responsive layout on your old WP site. There are many plugins like which you can use.