need a place to stay as close to a beautiful beach as possible on Crete .

Chania in W. Crete is a much better location to stay than Heraklion. Look at Hotel Ammos for a beachfront location. We didn't stay there but made note of it for a future trip. You can take day trips to Falasarna and Elafonisi beaches on west coast.

It would be helpful if you told us how long you have and if you plan to rent a car or rely on public buses.

If you are primarily focused on beaches then the south coast on the Libyan Sea would be your best bet. The village of Plakias is in Central Crete.

It has a blue flag beach with several other beaches less than 3 kilometers away. Lots of day trips available including the palm forest beach at Prevalli.
Plakias Crete https://www.flickr.com/photos/stanbr54/albums/72157665045541263

In the west head south to Paleochora or my favorite Sougia. You can actually do a south coast tour by Ferry and stay at remote villages some only accessible by ferry. Paleochors, Sougia, Agia Romeulli, Loutro and Chora Sfakion are all connected by ferry. This are has beaches hikes and mountain/sea vistas.
Ferry trip Libyan Sea https://www.flickr.com/photos/stanbr54/sets/72157653434073393
Crete Libyan Sea villages https://www.flickr.com/photos/stanbr54/sets/72157645487373222/

If you are into a more urban atmosphere then Rethymnon has a huge beach plus an old town area and a Venetian Fortress.
Rethymnon https://www.flickr.com/photos/stanbr54/sets/72157654700502270

Chania though would be my choice. While its beach is outside the city walls about a 12 minute stroll along a tree lined road its attraction is the old town and Venetian harbour. Day trips to Elafonissi beach and Falasarna are doable but may favorite is the boat excursion from Kissamos harbour 45 minutes west of Chania. This excursion is inexpensive and gives you two nice beaches and a pirate fortress to climb up to.
Western Crete http://www.flickr.com/photos/stanbr54/sets/72157632115757899/

Chania http://www.flickr.com/photos/stanbr54/sets/72157632117917828/

I concur that Chania is the best choice. I stay at the Blue Beach (bluebeach.eu) in Stavros on the Akrotiri peninsula. It's not fancy but it's cheap, adequate, and the majority of other travelers there aren't American. It's right on the water with a small rock beach on site and a three minute walk to a long sandy beach at Stavros. The peninsula has a variety of other excellent beaches as well that don't make the guidebooks; just ask locals. Chania is a few hours drive from Heraklion but a major highway, very easy, and a lovely drive (with wide swaths of nice sand beaches on the way--you could even swim or sunbathe before you reached your hotel!).

The south coast on the Libyan sea does have lots of nice beaches. My experience staying there, however, has been that wind tends to be more of an issue whereas on the peninsula it's pretty consistently low-key.

Both Almyrida (or Almiridha) and Kalives are small towns to the east of Chania (or Hania, or Xania) that have nice sandy beaches and provide reasonably priced holiday accommodation (hotels, or rental apartments). can't make hotel recommendations as I stayed with friends who lived there. My choice would be Almyrida. Good beach and swimming, wear beach shoes though as there are the odd rocks and sea urchins lurking in the deeper water. Lots of cafes and tavernas along the shore. A couple.of small groceries if you decide to rent accomodation and a really good fruit and vegetable shop selling local wines, olive oil, herbs etc. There is bus service into Hania.

Aptera, between Hania and Kalives is an interesting archaeological site with remains from Minoan, Hellenic, Roman, Turkish and WWII periods and a stunning view of Souda Bay.

INgelisma it is VERY hard to give you good advice when you don't share at all how long you have in Crete. If it's a week, then all the advice about Western Crete is the best -- and if you cannot book AMMOS HOTEL - http://www.hotelsofgreece.com/crete/ammos/ -(ideal location but only 30 rooms), you could go just about 1/2 KM farther and try Corinna Mare - http://www.corinna.gr

HOWEVER, you say "in and out of Heraklion" ... does this mean y ou're just going there as a quickie to see Knossos, only 2-3 days? Then you want to stay near Heraklion but PLEASE not in one of those UK "holiday package hives" -- an almost endless row of concrete blockhouses for 30 Miles. A closer, less-known little beach has an unfancy accommodation known as "Mike's Place" --http://www.mikesbeach.gr ... right on a bus route from Heraklion, small family-run, beloved by one of my favorite Crete Destination Experts. Walkable to the next town which has all the party people, but this is quiet and pleasant.

But we cannot know what exactly your situation is.... share key details & you'll get the best advice.